The money works out about the same but 25 days leave (or whatever the standard amount is) just doesn’t cut it for me. The older I get, the more I value time over money.
Also the flexibility of doing so: if I need to take tomorrow off, I'll just let the client know I'll be unavailable for the day, and not charge for it. If they really need my work to be done that day, I can offer a substitute, charge the client, and pay the substitute.
This! When I have enough money to take a year (or more!) leave without pay, but you say "sorry, you've used up your annual leave", the only option I'm left with is quit.
And when it comes time again to get another job, that company that wouldn't give me more time off is not going to be my first choice.