Paintballs are trickier than just a water balloon. They have to be rigid/strong enough to survive the blast of co2 or compressed air that propels them, then they have to be soft enough to break on impact without harming the other player.

They also just aren’t that messy. I worked under the table as a referee at a Paintball place when I was 13, and we played such that gameplay didn’t stop when refs were doing paint checks. We’d toss people out if they were intentionally focusing on us but I got lit up probably 10+ times a day, every weekend for 2 years. My jersey and slider pants came clean in the wash every time, and to this day the only lasting blemishes are the shredded fabric on the knees/elbows/ass from when I dove and slid a lot.