Most languages are fluid and develop naturally- they borrow, they steal, they lend, they influence each other. Nobody set down to say “ok, this is how grammar works,” things just happen as they’re adopted. Grammar rules are always a compromise, a middle ground between the natural selection and evolution of language and actual construction. And as much as we try to pin them down, they warp and change before our eyes. Languages are. beautiful. Languages are alive.
But not conlangs, not in the same sense. Constructed languages don’t follow the same rules- they’re created with intent. They’re set up to have rules and aren’t the natural development of our need to communicate- they’re created. Sometimes as experiments, or fictional languages (like tlhIngan Hol, aka Klingon- though developed for Star Trek, it’s grown substantially since development, and for a while they were actually offering scholarships for linguistics students. Some are art projects, some are just for fun- and then some, like Esperanto, are made to be auxiliary languages to facilitate ease of communication across cultures- Esperanto in particular is really cool, it’s a language developed to help foster global understanding and peace in the 1880s.