Because many performers want their concerts to be accessible to their broad fanbase, and this is probably better for their bottom line in the long run, since there is sure to be animosity towards a popular performer once fans discover that only very rich people can attend their concerts.
If that's the goal, then active measures to deter reselling tickets should be used. Thom Yorke succeeded at this when I saw him earlier this year. All tickets were Will Call and you had to enter immediately after ticket pickup. Selling tickets in a way that puts fans in direct competition with well-resourced resellers doesn't help fans either.
