They are trying to control how fast the general population can get guardians to mythic quality.
Allowing this would make a huge advantage to people who can play a lot more and makes it harder to judge the time frame.
There needs to be a limit. And their limit makes it so the entire community (as long as you are earning max seals) should have it after about 6 months (if I remember correctly - I could be wrong). About half that if you were to pay for every guild store package every single week.
I think the reason they put that limit is to be fair to everyone (well on top of the obvious reason which is that they want people to pay real money in the guild store).
I can understand why people would want more seals, but devs have to think about their money and the economy and all types of players. The guardians are supposed to be a long term goal, so there has to be a limit.
Now, it would be nice if they had another cap for personal, maybe giving 500 extra seals able to be earned. But again that would mess up their timeline and economy. So we’ll see where they go with it but it’s a lot more complicated than just buffing up the cap.