Collusion isn't that hard of a problem to tackle when people are colluding at a poker table as they act very differently to when they aren't. Since the poker site can see all the hands this becomes visible very easily.

They obviously won't catch all people colluding but they will catch most of them. And just changing IPs won't help much when they ban your account/payment details and you have to arrange another fake identity that checks out at a payment processor regularly.