The zoology of my setting takes place on a mostly dry planet - it's certainly got rainfall, seas, and wetlands in certain ecoregions, and life exists there mostly, but some things still live in more desert-ified territory. After a major extinction event life has gone backwards a few times and ended up evolving very simple bodied megafauna, ones with decentralized nervous systems, no classic vertebrae, and simple organs - most of them are masses of rubbery tissue with "functional cell-regions." Among these simplicities are a lack of internal digestive systems. Most of them have spongy "feeding surfaces" that secrete digestive enzymes, sopping up what's dissolved off the surface. I've built a LOT of evolution around the use of external digestive systems, it's an integral theme to the food chain and I think I've nailed metabolic diversity in the setting, I just need a reason for it to stick around long enough. I am not looking for reasons as to why this wouldn't work (honestly, the folks on this site need to be a bit more creative.)
I am asking, what selective pressures would avoid favoring internal digestive systems in arid environments that would otherwise cause the enzymes to dry out?
Of course I want to solve the glaring issue as to why this would work and/or how to prevent moisture loss as a result of enzyme secretions. I'm currently working with with ideas around 0% or closer water content in their enzymes, though it might be difficult to make work chemically.