What do you get with ERC20?
Compare that with what you get with SMT. Yes they are different in that SMT does not cover the range of uses that ERC20 does. They create general use tokens where SMTs are specialized.
How does a company that creates a ERC20 token reward users for upvotes? What is the formula used? How do they know what will work best?
Plus with SMT you get the secure wallet system, retrievable accounts, the impending hivemind (which I presume will be in place by SMT release), and a way to track how much everything costs on the blockchain.
SMT are providing a package to those entities which have the specialized needs that Steem handles.
If one is creating security tokens for real estate, as an example, they might be better off on one of the chains you mentioned.