You are failing to consider the time value of money. Getting some nominal amount of money now has more value to you now than getting the same nominal amount of money later. If you invest an amount of money now and it grows into a bigger amount of money later, in a sense, you can think of this smaller amount now and the larger amount later to both have the same "value", even though their nominal amounts are different. (And conversely, you would much rather pay a certain nominal amount of taxes later than the same nominal amount now. So taxes paid later have less "value" paid than the same amount of taxes paid now.)
You wrote that you see the advantage of Roth IRAs, but not the advantage of deductible Traditional IRAs, so you might be surprised to hear that, on a mathematical level, if you start with the same amount of pre-tax money taken out of your income to contribute, and assume the same flat rate of tax at contribution and withdrawal, and make the same investment choices proportionally, and withdraw with no penalty, you are guaranteed to be left with the same amount of money at the end, no matter how much time has passed between contribution and withdrawal, and how much the investments have gone up or down in the meantime.
The problem with your example is that you are not comparing equivalent contributions. You are comparing the same nominal amount contributed, but the Traditional IRA contribution is a pre-tax amount, and the Roth IRA contribution is a post-tax amount, so they are not equivalent (one leaves less money in your bank account after taxes are done than the other). To compare equivalent contributions, you must start with the same pre-tax amount. So let's say, you want to contribute a pre-tax amount of $1000. Assuming a 25% tax, that is equivalent to a post-tax amount of $750. So you would compare a $1000 Traditional IRA contribution to a $750 Roth IRA contribution, as both would take away $750 from your bank account after taxes are settled.
Then, let's say it grows to 10 times the original amount over the years until you withdraw. For the Traditional IRA you have $10,000, and pay 25% tax to get $7,500. For the Roth IRA, it grows to $7,500 and you don't pay any taxes on withdrawal. The reason why those two are the same should be obvious -- both the result of being taxes, and the growth over the years, are multiplicative factors, and multiplication is commutative and associative, so it doesn't matter which order you multiply the factors in -- they are mathematically guaranteed to be equal.
This is despite the fact that you have paid a nominal amount of $250 in taxes in the Roth IRA case and $2500 in taxes in the Traditional IRA case. So it's not the nominal amount of taxes paid that matters. When those taxes are paid matters too. In a sense, you can say that the $250 of taxes at the time of contribution has the same "time value" as the $2500 of taxes at the time of contribution -- it's the amount the government would have if it invested the taxes the same way you did. We can also consider the time value of the money we invested -- since the money at the time of contribution can be thought of as having the same value as the money it grows into after an amount of time, paying tax once on the contribution amount at the time of contribution, is equivalent to paying tax once on the entire amount it grows into at the time of withdrawal -- in both cases, you pay tax once on the money.
Now, of course, in practice, there are many other factors that would make one or the other better. For example, the annual contribution limit for both is the same nominal amount, but the Roth IRA is a post-tax amount, so a Roth IRA has an effectively "higher" contribution limit than Traditional IRAs, i.e. for Roth IRA contributions close to the limit, there is no equivalent Traditional IRA contribution possible. Also, tax rates are rarely the same at contribution time and distribution time. And, tax rates are not flat, but progressive with brackets. Although contributions are limited to a few thousand per year and thus likely to be within the marginal bracket, withdrawals are likely to be bigger and dip into multiple brackets, so the effective tax rate of a withdrawal may be lower even if the marginal rate is the same. And Traditional IRA has other issues like taxable income could make Social Security taxable, and Traditional IRA has required minimum distributions, etc. But the point is, to a first approximation, Roth IRA and deductible Traditional IRA provide a comparable level of tax benefit.
Now if you compare this to a taxable account, you can see that the taxable account fares a lot worse. If you start with a pre-tax amount of $1000, therefore post-tax amount of $750, contributed into a taxable account, and it grows to 10 times, to $7,500, you would pay capital gains taxes on 9 * $750 = $6750. Even if it is a capital gains rate of 15%, that would be $1012.50 of taxes, leaving you with $6487.50. You would need a capital gains rate of 0% to match the advantage of Traditional or Roth IRAs. You can understand why this is worse if you consider that you already paid tax once on all the money you contributed when you contributed. Then, when the money grows into a larger amount later, this is money that grew from money that was already taxed; so, effectively, all this money has already been taxed once, time-value-wise. Then, when you withdraw, you tax a portion of the money (the "gains") a "second time". (It's taxed the first time from a nominal amount point of view, but we already know that point of view doesn't give you a useful picture.) And if you have an interest-bearing or dividend-bearing taxable account whose gains are taxed every year, that is even worse, because some portion of that money would be taxed, twice, three times, four times, etc. due to the "gains" (which grew from previously taxed money) being taxed every year.
