Fitting is a service, and services cannot be shipped. Searching for somebody to fit a car seat produces a very short list of local results, and the shop on it also sells the seat.
Constraints are how prams are chosen. A small boot, a narrow hallway, a flat with stairs or a need to fit two children in a particular car are precise problems, and the shop that answers them is chosen over any catalogue.
Checklists arrive early and repeatedly. Somebody four months pregnant searching what they need is months from spending, but they read carefully and remember who answered honestly.
Safety questions build the relationship. When a seat can face forward, what a recall means and how long a seat lasts are asked by anxious parents, and a shop that answers becomes their default.