Why does Computers - Amp - Speakers sound better?

I have a budget system. Denon turntable, Shure cartridge, NAD integrated amp, and Wharfedale speakers. For some reason, computer -> amp -> speakers sounds SO MUCH BETTER than turntable/cartridge -> amp -> speakers, even when the source file on the computer is 256 kbps mp3.

I don't know why this is. It would make sense if my amp and speakers were leagues above my turntable or if the source file was in 320 or FLAC, but that's not the case. 256 kbps should not sound better than high quality vinyl of music from the past five years (i.e. not mastered from bad tapes).

Is my turntable or my cartridge (or both) inferior to my other equipment? Also, I'm using Monoprice cables for my computer ---> amp and off-brand cables for turntable ---> amp. Could that be why?

thank you. The reason why this bothers me is because I spent over 1000$ on my vinyl (not just for the better sound, but because I'm deeply involved with the new-wave punk scene) and $400 on my turntable/cartridge and it bothers me that a low-quality file produces better sound.