because he can.
If you wake up in the morning and you 're finding 500T or 1000 greens in your account, why not ?
Here 's a guy I 've met about a year ago.
He built Bernabeu, with 260.000 seats at lv7 when you don't have enough fans to fill it
About treble, you 're right. I won 6 times the treble with a team of 6* (and none or few early 7*) players.
With my 2nd team, I won the treble at lv3 with only 5* players.
But if you can easily build a 8*-10* team, why not secure it ?