They don't but they didn't increase capacity in order to have seats empty. Some of those clubs shut whole sections to reduce match day costs.
I'd love to be in a stadium full of 35,000 or even 32,000 but I'm not convinced there is a demand. OK for half a dozen games in.a PL season but the stand won't suddenly have a relegation clause like players contracts.
When I ran Bayer Club for all those years I soon realised that it was folly to run the business on the strength of what people said they would do. Talk is cheap. Much better to make decisions based on what people actually do. So forget the chatter right now while the sun is out and look at the evidence of seasons past.
2017 FA Cup v Southampton. There was outrage because the tickets were priced at £25 for adults £10 for U18s and £5 for u12s. Attendance 12,000
2018 FA Cup v Chelsea. Lesson learnt and prices dropped to £15 adults £5 U18s. Attendance 23,000.
2019 FA Cup v Portsmouth. Prices £10 adults £5 U21 and kids for a quid. Attendance 23,000.
So looking at what folk actually did shows that it was all about price. And yet those Southampton tkts that were too expensive were much cheaper than seats in a new stand would be.