Why do chassis need a front plastic panel? Wouldn't it be better to have just a few buttons on the front panel in the optical drive bay? Hard disk and so on designed to be plugged in like a server?

Home computers don't need the ability to plug in and out hard drives directly, so they don't do what you're talking about.

The front is not a plastic panel, the standard design is a metal front panel integrated with the chassis, with a plastic panel is simple material, if it is open, can not say what will get into the chassis.

The server chassis is what you said it looks like, with the hard disk plugged straight in.

If you don't design the optical drive bay, the chassis is easily unsellable. If you look at internet cafe cases, some of the buttons are designed right into the optical drive bay.