To a first approximation, nobody needs or wants 5¼” bays in their PC case. PC software is distributed almost exclusively over the internet and has been for years. Installing your OS is just faster and better from a flash drive. Motherboards have decently sophisticated fan controls and temperature sensing abilities that render the old style of fan controllers redundant.
On the flipside, PC hardware is increasingly power hungry and cooling requirements have increased, so dedicating as much of the surface area of the case to air exchange just makes a lot of sense.
We should also remember that making a drive bay is a non-trivial bit of fabrication that bears a cost, and when you're trying to compete in this market, everything that has a cost has to justify itself in attracting sales. If it no longer attracts sales (see paragraph 1!) then it's going to be removed. This is true no matter the price of the case, there's always price pressure at every level.
Also, DIY PCs tend to be built by people looking for the latest and greatest, the most modern, etc. Optical drives just aren't that, they're viewed as a bit of a technological relic by many and realistically, they have a point.
Lastly, USB optical drives are inexpensive and widely available for people who need them, so it's not like it's a capability that's been completely lost from PCs.
www.tomshardware.com
I still use my DVD Burner to watch DVDs from time to time. I know my dad still does.
