Mainly because it is much more convenient and fast. You just pop it in, pee, rinse it off and wash your hands right there.
If you were peeing in a toilet you would have to aim, check if your weenie’s stuck in sprinkler mode, and walk to the sink afterwards. Imagine what you could’ve spent those extra 4 seconds on.
Another great thing is that it saves plenty of water! You use 4.5 - 11L per flush where I live. Instead of wasting all that water you could just be doing a quick rinse, plus washing your hands cleans the sink too.
I only do that when I have a boner because it uses more water. Honestly though it’s just more fun to pee in the sink. Also sitting down risks masturbating unfortunately.