I noticed the same thing when I bought a couple of those things for the jukebox: Timebox and the Quik. Terible, thin sounding and not all dynamic.
My thoughts are back in the 70s mastering vinyl was the norm, so as long as you had decent source (even a good condition 45), you should have been able to create a decent enough sounding copy. Some of the worse offenderts from the 70s were clearly bootlegged from low quality tapes and so on.
It seems likely that, as Mace said, the new crop are done cheaply and I guess one way of saving cash is not spend money on using experienced people at the mastering stage, they will be few and far between now anyway.
Same thing goes for those awful 'carvers' that were doing the rounds until recently. Mastering is an artform - you can't just stick an MP3/WAV onto vinyl and expect it sound ok.