Because 1 day tickets are tiered and multi day tickets are not. Sometimes two 1 day tickets are cheaper than a 2 day ticket. Sometimes even three 1 day tickets are cheaper than a 3 day ticket.
The threshold is if both days you want to go are a tier 3 or higher, then a 2 day ticket is the better value. If one of those days is a tier 0, then it will be cheaper to get 1 day tickets no matter what the other day is.
It is more expensive to add a hopper to a 1 day ticket, though (the above only considers one park per day). For a 1 day it costs an extra $65 to add a hopper to it. It's $60 total for the whole length of a multi day ticket. And no, that's not backwards. $65 for 1 day, $60 total for length of multi day.
So if you were wanting say a hopper for both of those days, it would be cheaper to get the 2 day PH. But yes, those exact days it is cheaper to buy two 1 day, single park tickets. Also still cheaper if you only wanted one of them to be a hopper.
Just need to run the numbers of what you want.