Yeah. But most people “write” online like they speak…
That’s a common misconception.
While your written and spoken varieties do interact a fair bit, no, people don’t “write like they speak”. Not even online.
And that is not simply an “ackshyually”. A lot of AAVE features simply don’t transpose into writing - like prosody, non-rhoticity, /ɪ/-breaking, /äɪ/-monophtongisation… at most you can consciously approximate them into writing, but they won’t be there.
If people followed rules about language, yeah, vernacular would just be spoken speech. But that’s not how it works. The rules are made to reflect what people are doing.
That is not about people following/not following “rules”, it’s about nomenclature - it’s exactly the reason why “AAE” and “AAVE” are necessary as separated terms.