One of the biggest reasons you see acquisitions is for realized gains for investors (including the founders). When you seek investors for your business, you'll find that they expect you to have an exit strategy; this is because they eventually need to get their money out (and then some), and an acquisition is one of the best (and easiest) ways to do that. Some investors may even build legal terms into the agreement governing your course of action when an acquisition might be possible.

For most founders, the acquisition is typically the end of the rainbow. You quite literally cash out/sell your investment. You stop assuming the risks of the venture, but you also give up your rights to future profits. Startups are very risky, and like any investment people gain the most by selling when times are good. A startup that is doing fine today could be dead in two years, or it could go on to be Google. Would you rather have $30 million today or a chance between $0 and $1 billion+ a few years from now?

Of course you don't have to be acquired. If you like what you are doing you can simply keep going (if you are profitable). If you have investors then you'll need to figure out an alternative way to compensate them.

As a founder I personally tell you that I'm more interested in building my products and business than I am running the business long term. I love my company and everything we are involved in, but the further we go the further I get away from my hacker roots. Someday I'll need to get back to that.