The preamp section of tube integrated amps is usually pretty weak - like a 12au7 or 2, ugh. The volume control quality is also crucial, and often compromised here compared to what you’d get in a standalone pre.
In some ways it seems like a good idea - eliminate an interconnect w/ contact connections, and the preamp stage can be designed exactly for a known load. But the reality is it takes away real estate and budget from making the power section as great as it could be. A high end integrated could be done better with a larger budget but that’s not usually the target market, and so integrated amps usually file in towards a line’s entry level offerings, compromises and all.
Try a tube preamp like a Reference 6 and see what the fuss is all about for having a dedicated tube preamp with lots of tubes, real estate, and engineering to get this stage right.
Then there will always be an argument from the passive pre guys, saying you don’t need the gain from an active stage and it's all unnecessary coloration. Or that you only need a few dB at most from a very simple circuit, buffer, or transformer. I dunno - maybe that’s ok for a digitally sourced system. I only run vinyl and I need the extra gain available, and love what a good dedicated active pre does sonically.