Because it isn't an improvement, much like how getting someone who spells it 'color' to spell the word 'colour' or vice-versa.
Or should I say 'irreinterpret it'? After all, you would 'know what I meant' if I wrote that.
Ambiguity is fine if you're writing for poetic effect, but if you're trying to get a point across, it should be minimised.
And it does fit the pattern of negating the root morpheme.
[1] why not irregard instead of regardless?
Also, most English dialects don't strictly follow double-negation rules. I know this drives non-native speakers crazy, but you'll get over it with enough practice.