There are people (well, companies) who make money doing roughly what you describe, but not exactly. They're called "market makers". Their value for X% is somewhere on the scale of 1% (that is to say: a scale at which almost everything is "volatile"), but they use leverage, shorting and hedging to complicate things to the point where it's nothing like a simple as making a 1% profit every time they trade. Their actions tend to reduce volatility and increase liquidity.
The reason you can't do this is that you don't have enough capital to do what market makers do, and you don't receive any advantages that the exchange might offer to official market makers in return for them contracting to always make both buy bids and sell offers (at different prices, hence the "bid-offer spread"). They have to be able to cover large short-term losses on individual stocks, but when the stock doesn't move too much they do make profits from the spread.
The reason you can't just buy a lot of volatile stocks "assuming I don't make too many poor choices", is that the reason the stocks are volatile is that nobody knows which ones are the good choices and which ones are the poor choices. So if you buy volatile stocks then you will buy a bunch of losers, so what's your strategy for ensuring there aren't "too many"? Supposing that you're going to hold 10 stocks, with 10% of your money in each, what do you do the first time all 10 of them fall the day after you bought them? Or maybe not all 10, but suppose 75% of your holdings give no impression that they're going to hit your target any time soon. Do you just sit tight and stop trading until one of them hits your X% target (in which case you start to look a little bit more like a long-term investor after all), or are you tempted to change your strategy as the months and years roll by?
If you will eventually sell things at a loss to make cash available for new trades, then you cannot assess your strategy "as if" you always make an X% gain, since that isn't true. If you don't ever sell at a loss, then you'll inevitably sometimes have no cash to trade with through picking losers. The big practical question then is when that state of affairs persists, for how long, and whether it's in force when you want to spend the money on something other than investing.
So sure, if you used a short-term time machine to know in advance which volatile stocks are the good ones today, then it would be more profitable to day-trade those than it would be to invest for the long term. Investing on the assumption that you'll only pick short-term winners is basically the same as assuming you have that time machine ;-)
There are various strategies for analysing the market and trying to find ways to more modestly do what market makers do, which is to take profit from the inherent volatility of the market. The simple strategy you describe isn't complete and cannot be assessed since you don't say how to decide what to buy, but the selling strategy "sell as soon as I've made X% but not otherwise" can certainly be improved. If you're keen you can test a give strategy for yourself using historical share price data (or current share price data: run an imaginary account and see how you're doing in 12 months). When using historical data you have to be realistic about how you'd choose what stocks to buy each day, or else you're just cheating at solitaire. When using current data you have to beware that there might not be a major market slump in the next 12 months, in which case you won't know how your strategy performs under conditions that it inevitably will meet eventually if you run it for real. You also have to be sure in either case to factor in the transaction costs you'd be paying, and the fact that you're buying at the offer price and selling at the bid price, you can't trade at the headline mid-market price.
Finally, you have to consider that to do pure technical analysis as an individual, you are in effect competing against a bank that's camped on top of the exchange to get fastest possible access to trade, it has a supercomputer and a team of whizz-kids, and it's trying to find and extract the same opportunities you are. This is not to say the plucky underdog can't do well, but there are systematic reasons not to just assume you will. So folks investing for their retirement generally prefer a low-risk strategy that plays the averages and settles for taking long-term trends.