Why cheat the system?

I've been DMing for several years now, and nearly every group I've pulled together has had some player that tries to intentionally cheat the system. Some recent additions to this group of knuckleheads:

Oh we're counting ammo (3 sessions in to a resource restricted campaign)? My PC has a 20 in their casting stat now (level 4 with 27 point buy, impossible). Adds 5hp at level up after rolling for hp and getting a 2. Yeah im swapping infusions known (did not level up). Oh, no I haven't been tracking my spell slots, let me try and remember what spells I cast the past 2 weeks. Yeah my wizard's AC is 13, he's got a shield on (not proficient and has been casting spells).

They have a single character to track and their excuse is they didn't know, or no excuse at all and just a weak apology. I'm tired of having to police my players, and have taken a zero tolerance policy on future rule breaking/ignoring.

For those of you that are willfully ignorant of the rules or intentionally pulling the wool over your DM's eyes: why?

Edit: Seems the majority of responding cheaters are cheating because they do not like the rules and dice determining their story.

To those of you that are cheating - You are ruining the game for the other players at the table that are following the rules everyone agreed to. Either play a different system or join a table that you can agree on the rules of. Or not, I'm not your mom.