I'd say the main reason is because of what they are. Pure quality.
They're neve or SSL or whatever, but those guys are looking at 96 channels of gear, and each channel is probably worth a few grand.
The eqs and compressors and all that jazz; Same reason. Sheer quality, or unique contribution to the sound.
If you need gear on that scale, what's the alternative? 12 sets of high quality converters and either 12 boxes of 8 preamps, or racks upon racks of single/dual preamps?
A big mixer makes sense if you're recording the london symphony or something.
I'm going out on a limb, but I reckon if behringer made a 96 channel mixer with all the same functionality as some mad neve or whatever, no body would buy it.
Keep in mind, most/all of these consoles come with, or are retrofitted with hardware automation, so there's one drawback of the 'old days' dealt with.
They work like a massive 3d protools, or rather, protools works like a software version of these mixers.
There's the routing too. We're talking about studios that have a few hundred thousand tied up in outboard effects and preamps that are like hens teeth.
I suppose it's much more convenient to patch that into a traditional mixer setup.
Plus, many of these guys track to tape, then mix, then bounce to stereo tape. I'm sure there are many studios that pride themselves on being pure analog.
There's the control element too. It's just pure convenience being able to mute/solo by hand, move several faders at once, etc.
I can understand why a lot of home studio guys have euphonix control surfaces and things like that, but that's a lot of money for something with no sonic contribution.
All that all being said, I'm sure there are a lot of 'industry standard studios' who are ITB all the way.
Bottom line, on a budget of a few hundred dollars, a mixer is likely to do more harm than good to any signal path.