Why don’t tube amps sound like tubes anymore?

Its always tough to generalize, but the long term trend has been for solid state to less harsh and grainy and tubes to less lush and syrupy. Or solid state more full and rounded and tubes more tight and clean. However you want to put it.  

Why is probably because most of us are trying to get to the truth, and so over time the two approaches are bound to converge.

This is why i switched to solid state years ago plus i hated tubes going out and taking out tweeters.

Amp technology in general has improved over the years and more tend to sound better and hence more similar as a result.

Not to say there are not still other flavors of tube amp to choose from besides vanilla.

Because the extreme high end wants a certain sound and imaging that has changed over the last several decades.

The current trend is:

Exaggerated high frequencies and etched sound = "detail", biting unnatural attacks = "fast transient response", and unnaturally dry bass = "taut" and “tight”

There is something else to consider. The new valve preamps and amps have heat limitations.  They just can't get as hot as the old and be sold, as a retail unit.  The really great sounding valve units made today, are borderline with the heat issue, and probably tip the scales in the summer months..
Production runs of 5-50.  Not 500 to 5,000. 20,000.00 vs 3-5000.00
that 20,000.00 (sometimes) will get the "tube" sound back... The 3-5,000.00 model will keep the heat out, the sound., maybe.

The new VTLs might as well be SS they sound so, SS..LOL
Really, they remind me of the older Krells.

Regards