Why do so many begin questions in Yahoo Answers with "So" or "OK so" ?

Because a lot of users don't have good communication skills. They post as they would speak (and if they were speaking, beginning a sentence in that way would indicate a level of discomfort talking to others or in front of others).

English is apparently being taught very differently in primary school nowadays, than when I attended (in the 1970s).

So I don’t know

People write in the same way they speak. Most yahoo answers users are not well educated. They think it’s cute, in the same way they think it’s cute to spell school “skool” and was “wuz”, as you did here

https://answers.yahoo.com/question/index?qid=20180... 😂🙄

Because that's the way they want to start there Question's

it's a lack of education at school, so they are limited to what words they can use in a sentence