This is ultimately a supply and demand problem. If tickets sell out on the secondary market for 10 or 100x the face value, then that's the fair market price. Either artists should charge more, or perform more shows.

The last/marginal ticket in the venue sells for 10x face value. The majority of tickets don’t sell for much more than face value.

Taylor Swift can’t realistically play more shows than she did during the Eras Tour, and it’s unlikely that she’d have sold a million seats in London if she were charging much more than she did.

I think its more complicated than that. An artist is pretty constrained by how many shows they can play in a given area which makes the total market for any given show really small and trivially manipulated for profit.