What prevents mortar from being compressed with the weight of courses of brick?
Mortar is used much thicker than cement and even thicker than thinset.
Time
Bricks aren't that heavy and generally you aren't just stacking one on top of another.


The only time that you would ever have a weight issue is when building a column or post and if you were skilled enough to climb rows so fast that your previous stuff has no time to set.  I am not that good so I have never had that issue.  It takes me 3-4 minutes just to butter each brick even.  I imagine someone who is really good gets a lot of downtime when doing a post or they do a few posts at a time.

Also I imagine that someone asking this question has too much water in their mix.