It's always been like that
Edit: Also to add on to this. Of course "we've always done it like this" is not an argument for why it's good.
The thing is, the final four are usually held in a different, bigger arena from the rest of the tournament. Booking such an arena is expensive, and so you'll want to minimise the time you have there.
Also, matches on three days in a row have not really proven to be a real disadvantage for teams. On the contrary: we've seen plenty of lower final winners win the grand final as well, despite the map ban disadvantage.
That deep into the tournament, it's less about prep and more about resilience, form and momentum. Winning that LF and then playing the GF on the next day can actually be quite the advantage for a lot of teams.