system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the verb "to be" seem to be really irregular in a lot of languages?

Maybe this isn't even true, and it's just been something I've noticed in the small number of languages I'm aware of.

Edit: Wow, thank you everyone so much for your responses! I just randomly had this thought the other day I didn't think it would capture this much interest.  I have some reading to do!
gpt
> ...high token frequency correlates with irregularity (Bybee, 1985; 1995). As Bybee notes, isolated morphological exceptions require high token 
frequency to be effectively accessed; low frequency irregulars are more likely to be regularized, presumably because they are not sufficiently entrenched. But this fact should not be misconstrued to entail that the converse holds: that high token frequency necessarily inhibits generalization. ... In the case of morphology, high frequency forms 
likely receive little internal analysis, as Bybee proposes. (This is possibly due to the fact that high token frequency leads to reduction, and reduction leads to internal opacity.)

 -Adele E. Goldberg. 2009. Constructions Work. [Response] Cognitive Linguistics. 20 1: 201-224.

who in turn cites

Bybee, Joan   
1985 Morphology: A Study of the Relation between Meaning and Form. John Benjamins Publishing Company.   
1995 Regular Morphology and the Lexicon. Language and Cognitive Processes 10, 425-55.

Basically, high-frequency words (like the copula) are more likely to be resist regularization, and thus to be *preserved* from older forms. This makes them irregular in a new paradigm.