Since I'm a great fan of two of these (PT and Katatonia) and like the other two (Anathema and Opeth) a lot, I've noticed this as well and have to agree.
These four bands rely a lot on atmosphere, and even though all four have very strong progressive elements (well, PT definitely classifies as progressive rock, Opeth as progressive extreme metal; Anathema and Katatonia are less obvious but still have a lot of this) and also have quite complex songs - it's not on the same level as DT, which works exactly the opposite, as they are based on complexity above all, and atmosphere also matters but it sort of comes second. I guess some people just can't notice it and they see only technical complexity. Probably the most common argument I've ever seen (as long as the person is writing in a civilised way) in this case is "they're excellent musicians but they can't write songs".
I don't know if it comes from laziness, trying to look deep and atmospheric or just not enough attention, but it often seems to be like that. It's kind of weird because as you can see an important part of DT fanbase really likes the above bands (I'd guess around 90% DT fans love PT as well, and around 2/3 love Opeth; Anathema and Katatonia aren't that popular but also have their fans), but it doesn't exactly work the other way around. I don't like it at all, and to be honest I'm tired that whenever I read any board dedicated to a band which is somehow related to DT, I see exactly the same "instrumental wankery blablabla" comments whenever the name of DT appears. I guess all I would need to shut up most of them would be a link to Vacant or Trial of Tears; but it's like a battle against windmills or something, it seems to be rooted somewhere deep and many people just won't look beneath the "technical" shell. Which is sad...