I think there are many different reasons.
1. guys may not have the courage to talk to girls in the pub/club, so it's easier to just pay for getting laid.
2. There's a great number of really hot escorts - the type of girl many men think they'd never be able to shag otherwise
3. Men are generally polygamic - again, it's much easier and much less problematic to pay for the odd punt, rather than trying to juggle with keeping a mistress secret from their partner
4. We always get horny on the booze - calling an escort is a quick and easy fix for that
5. you can get certain services from an escort that your partner might not be willing to do
There are many more reasons, but I think the above probably covers about 3/4 of all punters
Life's not short enough - go figure!