Mostly because mod_rewrite rules are fairly complicated, and the ability to put them in the server's conf file means any changes require a server restart. You

Mostly because mod_rewrite rules are fairly complicated, and the ability to put them in the server's conf file means any changes require a server restart. You could put them in .htaccess, but now your having to parse out the route every time. Using a front controller (like index.php) makes everything so much easier. Finally, you have to consider hosting support. This hasn't even touched on numerous other benefits. Yes, their are tradeoffs, but they don't overcome the benefits, not the least of which is simplicity.