I have several reasons I can state.
1) Good revolvers and 1911's are deadly accurate. I consider most of them target accurate. Many modern semi-autos are "combat" accurate, which just means it will get in the ballpark.
2) 1911's are customizable. Just like a good muscle car of the Chevy Camaro and Ford Mustang era, people love to tweak their 1911's to make them their own. It's an open design that allows for many improvements and changes. While an HK USP or SIG P220 are good guns, they are just that. Wanna change them? OK, how about a trigger job and refinish. 'Not much else. What other parts can be changed? (Maybe mount a scope.) To use a computer analogy, they are not "open standards."
3) Revolvers are very collectable, especially S&W and Colt. This adds to their desire. Many different shapes, sizes, special runs, variations, engraved editions, barrel lengths, etc. Some people (like me) can live on one or two 9mm guns and be happy. What do I need more for? But I can't get enough revolvers in my stable! 'Too many variations.
4) From a pure aesthetic standpoint, revolvers and 1911's are beautiful. Show me any other design that is as nice on the eyes. That, along with all the custom and variations, makes them more collectable.
OK, so most of those reasons are for collecting rather than carrying/shooting them. But they are also fun to shoot! I'd rather shoot my revolvers than any semi-auto.
-Robert