I do get this.
Or maybe I don't. It doesn't bother me that the art is always the same. One thing that's great about Ryan North is his ability to keep me from noticing it until he wants me to notice it for a joke.
Its also pretty amazing how much variation he gets out of it. Sometimes its two comics for the price of one. Sometimes half the comic is T-Rex's imagination. Sometimes the setting is changed with a simple caption like "Meanwhile in Tudor England" (because the art, while repetitive is minimalist with almost no background elements) and there are several recurring off panel characters (as well as the occasional sprite added like T-Rex's imaginary Batman.)
Beyond that, as others have pointed out, T-Rex is a well conceived protagonist and someone a lot of us connect with. He's clearly trying to be fun and exciting, he's full of bravado and yet is so transparently insecure. One seems to come with the other. I think we all see that if he'd just allow himself to be accepted by his friends for who he is (and they clearly do most of the time) he'd be happy, and yet its so hard for him to do. Probably because deep down he doesn't accept himself as being good enough. Sure he's a T-Rex, probably the biggest ever, but he's not a Ninja T-Rex and that's just eating him up inside. I think he needs to be admired because its the only way he'll ever feel secure in the knowledge that others will always like him.
Aside from that, its a pretty educational comic. Half the time I'm learning something new.
Oh, and its funny. Really funny. Does that help?
Last thought. The last panel of clip art is just gold. It captures a range of T-Rex's personality beautifully and its great to put that energy right at the end. I especially like the various instances where its left blank because its so funny and yet so sad at the same time.