Explain why apps like FB/Shopify are more than CRUD applications?

distributed systems are inherently hard to build no matter how simple the idea is. consider a simple application like pastebin. the idea is a block of text and a shortened url associated with it. most people have built one. but only few have built something close to pastebin.

size makes all the difference. even if your version of pastebin promises 1 successful save per second, you can only get 86k saves on a good day!

if all your saves were unique visitors it means you are only promising 86,000 unique visitors per day. and that's only considering saving new snippets, we haven't considered displaying them yet.

that's essentially what these systems have to deal with plus security, backup & replication, monitoring etc...

There's a lot of detail you're glossing over. For instance calculating the total tax on a purchase depends upon, sellers location (to varying degrees of resolution down to county), delivery method, buyer's/store address, type of item (HS codes), maybe whether it's permitted, and probably many things I've never heard of.

[I work at Shopify but not in this area.]

Edit: saying CRUD app without discussing details is like calling everything 'just' a Turing machine/program with bits on a tape. It's true but doesn't consider complexity.