A few years ago, I would have said that the general observation of the OP is true: it's easier to earn large quantities of airline miles than it is a similarly-meaningful quantity of hotel points, regardless of whether you personally value hotel points or airline miles more.

Three reasons:
- Elite bonuses were 100% or more on midlevel tiers like UA 1P. Plus various route bonuses and seasonal bonuses were much more common. Hotel elite bonuses, even for high tier, were usually in the 50% range or less.
- Airlines were out in front of the hotels when it came to BIG credit card bonuses. My first 70k airline signup bonus was the DL Amex in 2000 or so. The hotel cards have been out there for a while but have only really ramped up the bonuses and annual renewal perks in the past few years.
- Airlines have always had many more partners than hotel programs. Both other airlines and other random products like flowers, rental cars, banks, etc. Some of those now do hotel points as well, but the best bonuses and promotions have usually been for air miles.

But I'd say the gap is closing a bit these days. I find it pretty easy to earn lots of both.

I personally value them both very highly. Sure, anybody can survive in a small seat at the back of a metal tube for 12 hours. And sure, I can always camp, stay in a hostel, or (in a fairly limited part of the world) use Priceline. On some types of trips, I might even look forward to doing exactly those things. But on other trips, I'm really happy I have points and miles to book J/F seats and a high-end hotel that takes good care of its elite members. I don't see why it has to be an either-or thing when collecting both is pretty easy.