What tournaments have been invitational lately? Almost every big tournament has a qualifier for at least 50% of the total available spots.Using ATP rankings would be fine if majority of tournaments were open qualifiers as opposed to in our scene where many past tournaments have been invitational. So the rankings become a self fulfilling prophecy.
What you said would be true a couple years ago, but I don't think it's correct anymore.
Looking at the upcoming and latest 1v1 S-Tier tournaments:
RBW Legacy: 50% qualified, 50% invitations based on tournament performance
RMS Cup 2: 100% qualified
Resurgence: 100% qualified
WWC: 100% qualified
KotD4: 50% qualified, 50% invited
Holy Cup: 50% qualified, 50% invited
RBW V: 21% qualified, 79% invited based on tournament performance
Even the tournaments with invitees generally have them invited based on performances in a previous iteration or on general tournament performance.