There are a few reasons why I do not add reverb and/or tremolo in my amps. When these effects were first added to amps, it made a lot of sense. No one really had pedal boards as a standard and having some onboard effects was very useful. These days most players have a pedal board, and there are a lot of options for reverb, delay, tremolo, boost, overdrive, etc., making the on board effects a bit obsolete in my opinion. Players have a lot more to choose from these days. There is also the cost associated with adding the extra circuity to the amp. It takes longer to build the amp, and it costs more to add this extra circuity to the amp. This cost gets passed on to the consumer, making the overall price of the amp higher. My thought is that money could go to better use by buying some great pedals. Finally, the more things you add to an amp, the higher the probability that these circuits can develop a problem over time. Think about an amp with channel switching, extra switch foot pedals, reverb, etc., there is just simply more possibly of those things developing an issue. I prefer to keep the signal path straight forward and allow the tube tone to blossom.
