You kind of can't have a comics universe without crossovers. Seriously, Superman is kind of a big deal, y'know? There's no way that Superman isn't going to be well-known in the DC Universe. Same with most high-profile heroes.
Crossovers used to be a rare thing.
You don't read comics much do you? The Brave and the Bold, one of the most popular and famous crossover series, was published regularly for almost 30 years, from 1955 to 1983. That's not to mention the Justice League which started in 1960, and the Titans which started in 1964. Plus all the OTHER teams which are scattered. And this is just the DC universe, you've got Marvel's Avengers as well as stuff like X-Force for them.