Let's try again.
Steel resists a bunch of types. Steel is super effective against two types. I don't believe Steel can inflict status ailments, and even if it can, it has very few moves that can do so.
Poison resists a significantly fewer number of types. Poison is super effective against one type. How does Poison balance out with Steel, then? It has access to poison, which Steel doesn't have.
Factor in a few other types.
Psychic resists very few types. Psychic is super effective against two or three types. Psychic makes up for these low numbers of resistances and super-effective relationships by having a wide variety of support moves.
Electric resists a relatively few number of types, too. Electric is super effective against at least two types. But Electric is only weak to one type, and it has access to the (not-so-exclusive) paralysis status.
...and on and on.
When comparing two types' "overall performance", that means comparing everything
about them. Because offense and defense aren't the only aspects of a type, it isn't just unfair to assume Poison is worse than Steel because Steel is super effective against more types and resists more types--it's wrong. (And besides, Steel is weak to three types, whereas Poison is weak to two.)