If serrated blades cut better and do more damage than straight blades, why weren't swords made with even crude serrated blades?

I don't know where you got the idea that serrated blades cut better, because they really, really don't. They are great for sawing something, but not cutting, and sawing through something takes a lot of time. They certainly don't do more damage, at least not of the relevant type - when you're fighting someone, you wan to open up an artery, cut a tendon or sever muscles to stop him from hurting you back, and serrated blade helps with none of that. It actually makes cutting through something in one motion harder.

Then there's the problem of material, even modern steel has problems when it comes to "teeth" breaking off or bending. Pre-modern metallurgy would have an immense problem with durability of these swords.

And consider clothing, modern t-shirt has nothing on durability when compared to medieval tunic. Even a relatively thin padded armor, or just a somewhat thicker woolen tunic, would make a sword like this completely ineffective, since the teeth would snag on the fabric. Sure, it would hurt because a kilo of steel is a kilo of steel, but why even bother with edges at that point, since you're basically bashing them with a blunt steel bar?

There were some experiments with wavy blades (modern term is flamberge), from what I've heard of people who did some extensive-ish sparring with them, they work just like a normal sword, but have advantage in some types of binds. Sine they were rare, one can conclude that people who used them though that the slight advantage sometimes was not worth the extra time you need to sharpen them.

Oh, I didn't mention that: blades like this, be they serrated or wavy, are a royal pain in the ass to sharpen. I can be done with my arming sword sharpening in about 10 minutes, the guy who had a flamberge for test cutting kept griping about how it takes over an hour to sharpen it properly.