Challenges aren't the only reason lounges can be full. It can be for a variety of reasons:
* people booking exec level rooms & having legit access,
* properties 'upselling' to exec level at check-in (I've seen that at multiple chains btw, not limited to Marriott)
* where hotel is located; urban and convention properties will probably have more 'road warriors' who have legit status
* those doing challenges (who may decide to stay elite/w Marriott due to challenge or the 1-offs who need it for a vacation, but won't be staying around/taking up space at any other time.
* people bringing in all their colleagues and/or family (true whether it's challenge or road warrior)
I think Marriott is smart to give those doing a challenge the bennies while they're doing the challenge so they know what they'll get if they keep the status (and sometimes what they'll get is a crowded lounge
).
Cheers.