As both a web dev and a parks lover... have you tried to use the agencies' own sites much? Even for a flagship park like Yosemite (
https://www.nps.gov/yose/index.htm), it's hard to figure out how the different permits work. They're way down the page, requires going through several similar-sounding pages, all to no obvious next step. Even something as basic as having clear calls-to-action aren't there.
And figuring out where to go in the park... what's the difference between Plan Your Visit and Things to Do and Places to See? Why is it so hard to get a cohesive map that shows not just where to sleep but also the best places to visit and the recommended driving/hiking routes? What's the difference between a standard map, a brochure map, a simple map, a park map, and a valley map? Why aren't road closures and other alerts reflected on the maps?
Why doesn't the Fees page talk about permit or camping fees at all?
Which areas of the park are reservation-only, which are permitted, which are first-come-first serve?
These are just basic examples I pulled off one site. Most NP sites are like that. And remember this is for Yosemite, one of the country's (if not the world's) most visited tourist destinations, and they couldn't even do basic UX well. This isn't for a lack of knowledge (the on-the-ground rangers are awesome! talk to one next time before you venture forth!), but digital isn't a priority to them. It's not even an afterthought. It's more often than not "oh, I don't know how that works or who works on it or who pays for it".
Before Recreation.gov and ReserveAmerica before it, digital efforts WERE often done in-house, to disastrous effect. Even now, you will usually find better visitor's guides and resources outside of official NPS sites, like on Alltrails or random blogs or "best of" lists.
There is no magic sauce in web development, but it does take a particular type of agency to be able to make good pages with a focus on the end-user. Maybe to you and I such a site is relatively simple to set up. But that is NOT the NPS's strong suit at all, and it would be a forever uphill battle to convince their administrators and the Department of Interior and Congress to focus on usability. I doubt many of them have ever tried to book their own reservation.
Many of their parks don't even run their own restaurants and lodging[1] (which any tourist destination usually would), but you expect them to do well running a multi-agency, multi-site, multi-rule booking system? It's just not practical given the current staffing and skillset.
[1] If you think outsourcing to Booz is bad, yeesh, the hospitality concessionaires like Aramark are even worse. Talk about overpriced and poor quality =/