how come it is asking me for a cd

If you are using a retail copy, there is DRM which requires the disc to be in the tray at all times.
https://pcgamingwiki.com/wiki/The_Elder_Scrolls_III:_Morrowind#Availability

The GoG copy doesn't need a cd. I never had to use one. But the game may be looking for a cd drive as part of its startup process.... maybe? I would imagine that would have come up before if it was the case though.

The GOG copy of Morrowind has no DRM, and does not require a disc check. You mentioned rebuying; does that mean you originally had a retail version? If so, I would backup your save files, uninstall the disc version, and then install the GOG version. Installing the GOG version over a retail build may be the cause of the problem.

I once had this exact problem for a different game, I fixed it by finding the game in GOG Galaxy, right clicking the game, and pressing "verify/repair"