Being in the champions league regularly, means you have a far bigger income so can spend more money.

Which means you can continually stay there. (a few years of being outside and you will be in trouble, but it's harder and harder for other people to permanently dislodge you on the back of one bad season)

Also the teams who are cementing their place are the teams who were spending more freely before the rules came in. If everyone started from the same place then you could argue PSR would be alright, but Man U and Liverpool had spent millions for seasons and seasons. Everton got one crack at it (fucked it up completely) but couldn't reset and have another go in the way Liverpool did.