Quality of themes/support is not problem of Themeforest. Tons of independent or dependent designers/developers make and sell junk with same level of support. Some of them are better with support, some with quality of code, some with design, almost nobody with all these.

Majority of theme shops sell themes builded several years ago, many of them instead of developing just write blogs and do referrals for hosting companies ...
Price for theme is low and quality is on the same stage.

How @needforname mentioned ... I would pay $200 for quality theme, but these kind of themes does not exist.