Is it because 're is harder to pronounce, and adding -s/'s to words happens a lot in this language?
It's not hard to pronounce
-'re, but nonetheless, I think you're exactly right. It's not a matter of what's easy. It's a matter of what's easi
er.
There's and
how's are easier to say than
there are and
how are, so we substitute them, especially in conversation.
In American English, we don't write
there're or
here're because, spoken, they sound the same (or very nearly the same) as
there are and
how are.