Cary MB CAD 500 amp

They mainly do tubes. Why do they have to build on something that is this good?

I don't understand the question? Do you mean why have they 'stopped' building this product?

Macd, Typically companies build\make improvement on existing equipment. For instance, Cary could have "Cary MB CAD 600 amp" or "Cary MB CAD 1000 amp".
This may include better power supply, 600 or 1000 watts version, bigger/better transformer etc. basically an improvement over the initial version.

Why does a company discontinue a product? In some cases because a new design exclipses the old design, or it just didn't sell well in the first place. I can't answer this for sure in the case of the Cary CAD-500MB, but it could very well be because it didn't sell well. So, if that was the case, why would they build on it with a 600MB, 1000MB, etc? With that said, when I sold Cary products, I really liked the CAD-500MB. They were very good monoblocks.

Perhaps the two-channel audiophile wasn't convinced that a primarily tube manufacturer could make a good solid-state amp (which would have been a mistake), and perhaps the home theatre buyers didn't want stacks of monoblocks to run their setup. Just guesses.