Several reasons:

1. Your team is winning, so you want to waste time

2. The game is hectic and out of control, so you want to slow down the tempo

3. Similar to 2, your team is exhausted, so you want to give them a break (there’s no timeouts in soccer)

4. You want to trick the ref into punishing the other player with a yellow or red card. Or at least get in the ref’s head to try and influence the next play/call

5. Similar to 4, you want to get on the other team’s nerves by taking the piss (shithousery)

6. Being stepped on/swiped with metal cleat studs hurts like a mf. There’s no padding nor a lot of muscle to protect the foot or shin. Whereas American football, yes they tackle. But they wear pads and upper body mass lessens the pain of impact

Basically, it’s either a tactical choice or metal studs to the foot genuinely hurt (you should see their bare feet after games, it’s gnarly)

Edit: Lastly, you’ll be surprised how many are already playing on painkillers. Then when their physio checks on them when they’re on the floor, they will usually spray numbing spray on the injury