A very large portion of daily games are won by early time losses and having a minimum of 4 moves helps prevent some rating inflation.
Why does Chess. Com not award rating points when someone resigns within 3 moves?

but it is their fault they dont play.They can just waste time on purpose
Most such games are not in winning positions for the person with time. Also, a lot of members start daily games not understanding what they actually are. They're not wasting anyone's time on purpose and with daily, you can have a lot of games ongoing some games that time out don't really waste most member's time
I mean this is unfair. Someone can play f6 and g5 as their first two moves and resign before their opponent gets a chance to mate them. Or maybe a 14 days match is going you play the first move and then you don't play and make your opponent wait for the "win" that doesn't even give you points. Someone can do this on purpose also. The rule should one both sides have played one move, there will be change in rating. I have faced this a few times.