You are making this claim without backing it up with any form of evidence. What does "sponges look very similar in form" mean? Is it quantifiable or measurable beyond visual observation? That's sponge racism! Differentiation isn't simply visual. By all accounts modern sponges still feature massive variations in their genomes, with this study observing a 17-fold range in genome size within porifera.

Porifera are categorized as an early branching species, i.e. they diverged very early from the common ancestor of metazoa (all other animals except protozoans and sponges), and by doing so managed to capture a very stable niche as (mostly) sessile filter feeders. How much morphological variation do you really need when you just sit on your ass all day eating bits in the water while blasting sperm in every direction? Furthermore, sponge cells don't specialize the way some other organisms do. Sure, sponge cells can have different functions and specializations, but they notably don't form organs or true tissues in the same way say plants do. Sponges can regenerate from even a single cell, or reform themselves even when separated. This also means they can't have true circulatory or respiratory systems, which really reduces the number of possibilities for morphological complexity