There are a few issues with your assumptions, which significantly downplay the benefits broad investment vehicle you are calling 'pensions'. I will tackle 3 of them:
Tax advantages: this is jurisdiction dependent*, but you are assuming the initial investment is 'not taxed', and that the future withdrawal is. Let's keep that assumption, and assume a 30% rate of tax.
If I invest $1000 today, and get a $300 tax savings, and also invest that $300, and earn your suggested 7.3% for 30 years [1.073 ^ 30 gives us a multiplier of 8.3], then I would have 8,300 in a yet-to-be-taxed pension account, and 2,483 in an already-taxed account. Withdrawing my 8,300 at the same 30% rate of tax [I am ignoring that most withdrawals will happen at a tax bracket lower than when you made contributions, as most people earn more during their employment years], nets me 5,810 after-tax from the pension, and 2,483 from the non-pension. Remember that of that 2,483, all but $300 is earnings, so I need to pay some tax there too - let's assume the same 30%, which leaves us with 300+2,183*.7 = 1,828 [I am ignoring lots of possible tax complications because we don't know the jurisdiction, but this is the gist of things], which is $7,638 total after-tax amount to spend in retirement if we invest with the pension.
If I invest $1000 today, it nets me $8,300 at retirement, of which 7,300 is earnings, which we need to tax at 30%, meaning we are left with 1000+7300*.7 = 6,110. In this hypothetical example, the fact that our tax savings are compounded for 30 years makes them more valuable than avoiding tax down the road. You may wonder why my answer differs from what you seem to have calculated - remember that your earnings above your initial $1000 will be taxed one way or another; I have generously assumed that your earnings in a non-pension account will only be taxed when you withdraw, when actually taxes will probably be owing on interest, dividends, and capital gains along the way, which will further limit the compounding you get in a non-deferred account. [to quantify this extra cost, we can reduce your 7.3% earnings rate by 30%, to 5.1%, which after 30 years nets you a multiplier of 4.45x, meaning your ending after-tax amount could be as little as $4,450 in a non-pension account, assuming 100% of your earnings each year are taxed in that same year]
Side note on tax advantages - they may be available to you in other ways; for example in Canada RRSP's are entirely self-directed investment vehicles that have the same tax benefits as pensions, with more flexibility in some cases. This comes at the drawback of loss of employer setting up the plan + employer match if applicable.
Company match: This one is quite straightforward - If you get a 1:1 company match, then for every dollar you invest, your company gives you a dollar. That means you double your investment, right from the start, and get to accrue earnings on that doubling immediately. You simply can't beat an immediate 100% return without gambling on some zany get rich quick scheme.
Investment mix: If you have a defined-contribution pension plan, you are likely in control of your investment. You may have a limited number of options to invest in, but should easily be able to adjust your debt / equity investment mix in all but the most lacklustre plans. If you desire to go more advanced than what your plan can offer, that's where you should consider 'topping up' above your plan, and investing on your own.
In short, specifics of your plan and the tax surrounding it will impact its benefit, but if you get a tax savings and a company match, the result is basically unbeatable.
*I see you've added the UK tag now; I don't have the knowledge to convert the generics of my answer to the specifics of the UK system, but the general thrust of benefiting by compounding your initial tax savings should still hold true.