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.