If the compound words, “insofar,” and “inasmuch” require that they be followed by “as”, why haven’t we made the leap to “insofaras,” and “inasmuchas”?

Constituency.

If people are going to run words together, they don’t so randomly. They run words together when the words form a syntactic grouping. And the stop running words together when they run into a syntactic break.

A clause like “in so far as I am able” is analysed syntactically as:

[in [so far]] [as [I am able]]

There is a break between so far and as I am able. in so far, OTOH, can be argued to hang together as a group. (Even if it doesn’t, it can be argued to be reanalysed into a group that hangs together. Handwaving there, because I don’t care deeply about syntax.)

So there is a natural intuition that prevents you making the leap to insofaras: the as belongs with the following clause, so you can’t run it in with the preceding clause.

As to why the run in to begin with: that’s syntactic reanalysis. in so far as does make sense, if you think about it, but it’s a fairly abstract  kind of sense, using a metaphor with spatial extent standing in for validity. Once the metaphor becomes opaque, particularly in a legalese context, people won’t really make sense of in so far word-for-word; so they’ll be tempted to rattle it off as gobbledygook, and thus reanalyse it as a single word.

What they won’t do is extend the gobbledygook to the next word, because they do still understand that as is a conjunction introducing a clause, and not gobbledygook.