There isn't a very satisfactory answer to this. It's really just about orthographic convention and the punctuation guidelines that we're expected to follow (and that we expect to see) in formal writing. Commas are considered a "weaker" punctuation mark than full stops and semicolons, i.e., they are not considered sufficient to separate two independent clauses (clauses that can stand alone as full sentences). Would it cause confusion? Not necessarily, though it might make the sentence a little harder to read at first. The main issue is that, due to the conventions, we expect a dependent clause or a phrase to follow a comma, not another independent clause. So when we read a sentence with a comma splice (two independent clauses separated by just a comma), we expect that second clause to be dependent on the first, i.e., more connected in meaning and syntax to the first clause than another independent clause would be. So it may cause initial confusion/you may need to reread the sentence to see that it's actually two independent clauses.

But in general there isn't a great deal of logic when it comes to punctuation, and we can see this by comparing languages. In English, we do not use a comma before "that" when it introduces a dependent clause:

"I think that he is nice."

But in German, they do use a comma in this kind of construction:

"Ich glaube, dass er nett ist."

As an English speaker, I'm sure you see no need for a comma there, but in German it's just the convention to use a comma to separate any dependent clause from the main clause.