They did lol, and that's a really dumb question by a tech illiterate. Optimization isn't a boolean state, it's a constant ongoing process that still needs more work.
They only meant to say that Bethesda did optimize throughout the development process. You can't do gamedev without continually optimizing.
That does not mean, Bethesda pushed those optimizations particularly far. And you can definitely say that Bethesda should have done more.
Point is, if you ask "Why didn't you optimize?", Todd Howard will give you a shit-eating-grin response that they did, and technically he is right with that.
My entire comment is about why your response doesn't make sense, they do and it's not a process that's ever "done". It's whether how optimized is it and if it runs well on targeted specs.
Also optimization happens between a minimum and a maximum. If Bethesda defines that the game should have a certain minimal visual quality (texture and model resolution, render distance, etc), that will lead to the minimum that hardware has to offer to handle it.
What modders so far did, was to lower the minimum further (for example by replacing textures with lower resolutions). That's a good way to make it run on older hardware, but it's no optimization Bethesda would or should do, because that's not what they envisioned for their game.