Because at the current time, mirrorless isn't entirely "obviously superior". It's been rapidly improving and gaining market share (for good reasons), but there's valid reasons to still have DSLRs too.
Also, parts of the market aren't buying "a camera", they bought into an ecosystem that's more than just camera tech. Someone in the Nikon or Canon ecosystem (the two only(?) important players that primarily do DSLRs) will stick with that even if they don't do mirrorless versions of their high-end cameras.
The argument over DSLRs and mirrorless remind me of the endless SLR vs rangefinder debates among film users. Each side touts various advantages that exist more in their own minds than in the actual cameras.
As there are more and more rumors about Nikon and Canon finally intoducing professional mirrorless cameras, they certainly would also support all their current lens systems.
Sure. But they might be making the jump now, when they can be very competitive with the DSLRs, instead of earlier, when it would have been a "why bother buying a worse product?" offering. (+ of course likely some amount of corporate inertia)
E.g. a large part of why Sony is so attractive right now is because their sensors are the best: Canon making a mirrorless isn't going to have that benefit - and if they had the tech, their DSLRs would gain it too.
The sensor is just one thing. There are numerous categories of lenses where the Sony offering is either inexistent or completely overpriced vs what Canon or Nikon offers. Sony cannot win just everywhere, at least right now.
Hopefully they also take advantage of the shorter flange distance of mirrorless, to redesign their lenses to be more compact too.
Being able to reuse existing glass is great (in fact, that's why Canon / Nikon adapters for Sony's mirrorless are so popular), but it always ends up being suboptimal (sure you save some weight on the body, but the lens is still massive).
Do you have some examples of lenses with comparable optical properties that are smaller in mirrorless mounts?
AFAIK demand for corner-to-corner sharpness wide open has made lenses huge NOT whether there is a mirror or not. I have a bunch of old school Nikon primes that are tiny compared to modern lenses but are essentially "portrait only" lenses until f/4.
I think the Zeiss Loxia (mirrorless) and Milvus (SLR) lines are an interesting comparison point, since they're both from the same manufacturer, both manual focus, and should both be comparable quality.
The Loxia 21mm/f2.8 is 72 mm long, and 394 g.
The Milvus 21mm/f2.8 is 95 mm long, and 735 g (Nikon mount)
Once you go to a higher focal length, keeping the lens compact requires sacrificing something, i.e. slower glass:
The Loxia 35mm/f2 is 59 mm, 340 g.
The Milvus 35mm/f1.4 is 125 mm, 1131 g.
The Loxia 85mm/f2.4 is 95 mm, 594 g.
The Milvus 85mm/f1.4 is 113 mm, 1210 g.
So, I'd sum it up as, at wider angles, the lens designer probably has a bit more flexibility because of the absence of the mirror box, but at longer focal lengths it matters much less. It's also interesting to me that Zeiss is focusing more on slower lenses that retain the compact form factor. They do have a 35mm 1.4 FE mount, which is huge, and Sigma recently released their FE versions of the Art prime lenses (which are just as big as their SLR equivalents), but the bulk of the Batis and Loxia lineups are f2 and up.
Seems to indicate that while there was indeed a lot of demand for huge, extremely sharp f1.2 or f1.4 lenses in the SLR world, mirrorless customers generally seem OK with trading some of that speed for less weight.
Also, parts of the market aren't buying "a camera", they bought into an ecosystem that's more than just camera tech. Someone in the Nikon or Canon ecosystem (the two only(?) important players that primarily do DSLRs) will stick with that even if they don't do mirrorless versions of their high-end cameras.