Answer 1

IMHO probably if it was available, then SoloLearn would have to provide a SQL Playground where learners can test the lessons like they do with "Try It Yourself" feature for codes. And with the many users trying out, the SQL Playground would need a powerful machine to run the service on (more servers, more expense), otherwise the vast amounts of request/response traffic might take the server down.

Actually, if you want to know the real answer to the "Why", I suggest you to ask the SoloLearn development team directly, send them e-mail ([e-mail]), that way you get direct answer from their point of view.