There's absolutely no reason why you should have a month to claim a login prize. I've played plenty of mobile games that just keep that all for you. People come back from a break and they get a pile of stuff they missed.
Sorry, there's no GOOD reason to use this kind of system, other than to artificially force people to play every day/month (gold rolls could totally rollover lol). But like I've said before, it really seems like they're trying to copy battle passes from successful games, but not doing it right, at all. I bet the number of people that login more as vip than they would've otherwise is uh...zero? Ok maybe 1, someone whose PC broke or something and they couldn't get it fixed for a week or 2, and they hadn't picked up the VIP reward. They might have a friend login for them and claim it.
Contrast that to the number of people having issues claiming, forgetting to, claiming on the wrong person, etc. That's...well, at least 1 lol, but I'm sure a fair chunk more.