More accurately they didn't all, ALWAYS wear adorable hats. It very well could be the norm, just not when treking through the tar pits... or just coincidentally the .00001% that actually fosilized in ways to give analysis of soft materials happened to have left their hats at home.

In fact, we know that they did wear adorable hats made from compostable materials. It's just that the hats were extremely valuable, and so whenever we find dead dinos without their hats it is likely because of grave robbing, or potentially even that they were murdered for their hats. Held at claw point, and marched into the tar pits, all for the hats so they proudly wore.

Source: visions

Well, not all the time anyway. How many humans were buried with hats on?