IMO complex grammar just creates a high barrier to being understood at all, while hiding the complexity in vocabulary allows a more gradual learning curve.
Would it really be simpler if there were a language where the grammar were that you only say one word and one word only, but all expressions require you to memorize a dictionary of a billion words because they’ve decided they want their grammar to only be one word utterances?
The other extreme would also create a very high barrier to being understood at all.
Agglutinative languages say hi.
I think you meant polysynthetic languages. And you don't have to memorize all the forms of each stem, there are grammar rules that usually let you derive each form.