Why? It is a regular practice on different forums that you should meet some criteria before ranking up. Here activity freeze is used as a way of implementation of this. BTT for instance also has activity freeze and even if it is not so popular, most users are not so upset with it. 
It won't be bad at all if the post count was available but most people count number of posts by looking at the activity of the member and that becomes a problem when someone's activity is frozen.
I know that someone's posts can be counted manually by visiting that user's profile but it would be better if either post count was available on the forum or activity freeze was disabled.
That's true mate because I noticed this too, but I also feel if users are let to grow account on rapid rate it can also have negative effect because most spammers would just be after growing their accounts here because of the simplicity of it and possibly fill the forum with shitposts and spam.
You have a valid point but at least this activity freezing function should be disabled on the profiles of those users who make good quality posts and contribute helpful information on the forum.
The function should be enabled for low rank accounts only who might make spam posts in order to rank up but the high rank members aren't that much interested in ranking up and if they face that activity freeze then they somehow get hurt.