They don’t, actually.

The plastic on the handle is a thermoplastic. Every time you send it through the dishwasher, it loosens a bit each time. Eventually the handle will become a danger to use with no way to fix. Before that happens, small gaps form which will harbor bacteria.

Also the water spray causes them to bounce around and dull (or in the case of the OP’s ceramic, chip)

Finally, the harsh deferent used degrades the plastic. This won’t have any issues that’ll crop up before the first two become problematic, but it will cake the plastic to turn whitish

Basically, my parents agree with you and also wonder why my knives actually stay sharp.