I also don't use wireless mouse or keyboard. For me, it's mostly about having confidence that it will work - and that if it doesn't work then there are fewer points of failure to think about.

For example, if my keyboard isn't responding... that could be because the computer has frozen and become unresponsive, or the keyboard is broken. That's about it. But with a wireless keyboard I have to consider that the keyboard itself might be working but the wifi is not. Perhaps the keyboard's radios are damaged or turned off; perhaps the battery is flat; perhaps the computer's radios are turned off, or the drivers have failed; or perhaps some device is interfering with the signal. ... I find that these thoughts just fill my head with crap that I don't want to think about or worry about. In particular, I'm sick of dealing with batteries. The cable doesn't bother me, so that's a better solution.

I remember one time I was using a wireless device, and it seemed to work fine except when I was sitting at a particular desk. I spent ages trying to work out what was going on before finally working out that my computer monitor was blocking the signal. Adjusting the position of the monitor, or the device, or the thing it was talking to fixed the problem. Changing the wavelength also fixed the problem. ... Wired just doesn't have any of those variables.

I haven't had problems with my wireless mice but headsets seem to get worse the more money I spend (never above $200, granted).