>>105681892 (OP)Because back then all you needed was an idea. Facebook was literally invented by designing an home page(the feed) that was constantly updated to increase engagement. Instagram was invented by creating a Facebook clone but for photos and Whatsapp was a XMPP-based network application to overcome the SMS design limits.
None of those ideas were revolutionary from a math/CS perspective. Real computer scientists/mathematicians do other kind of things(algorithms, AI, cryptography, CV, etc.) which are rarely profitable.