If getting self-educated in the stock market is easy, why do people still grow their wealth through fixed deposits or SSB?

Fear of losing money.
A lot of friends I know want to invest only in safe, guaranteed products. They are scared to lose money.
It is a psychological barrier that cannot be learnt or taught. It has to be overcome from within one's own thinking.
Especially so for newbies doing their first trade with their hard-earned money only to see it drop in value. How to condition oneself to the ups and downs of the market is key.

Probably because they don't think it is easy.

Also lack of time, not interested in taking a risk, trying something they don't think they'll ever understand, fear from stories and accounts of people losing their entire fortune from investing in the right stock, etc.

Also, you don't have to invest in stocks. Some use property, bonds, insurance, and for those that want equity exposure, might just buy a fund from a bank or a financial advisor.