One house I lived in had the washer and dryer in the garage with no
vent for the dryer. I never had any humidity problems as far as it
didn't create mold or anything but you could tell it was more humid
when it was running. It didn't make any dust clouds in the air but
dust bunnies built up behind the dryer and workbench after a while.

Basic physics at work here.... Medium to lukewarm exhaust air with
lots of humidity and particulate pollution (dust) is a waste product.
You want to get rid of it. There is a bit of heat value available
for recovery but the quantity is low and un-economical to extract.

In most cases, a dryer only runs for 1 - 1 1/2 hours each day (if that
much) and by the time you add the inefficiencies of an air-to-air heat
exchanger, there is just not that much energy to extract.

The humidity that is potentially worth something is offset by the
expense of maintaining a filtration system that can clog very easily
with all the dust. (Look at your dryer filter... You are supposed to
manually clean this before every load). Dusty, moist air is not very
healthy in a sealed house.

When all is said and done, the most economical solution is to vent the
dryer exhaust to the outside, and this is why its been done that way
for years.

Beachcomber