Agree with others, it definitely is not your imagination. This is occurring for a variety of reasons. Most of it boils down to costs/lack of profitability.
The main issue with regards to profitability is the cost of school horses. First of all, a horse that is sound enough, reliable/safe enough, and has enough training and ability for students to learn on is a fairly valuable creature. If a trainer owns such an animal, it is almost certainly a better financial prospect for the trainer to sell or lease it.
Secondly, horse keeping expenses have gone through the roof. It is very expensive for a trainer to fill a stall with a lesson horse and foot the costs vs. filling that stall with a client horse that has its expenses paid and is generating just as much (or more) revenue in training/show fees. In addition, lesson horses get sick or need layups at the same rate as client horses, and it really is extremely difficult for a lesson horse to generate enough revenue to cover those bills.
Thirdly, many good school horses are older or have a minor soundness issue but there are very few socially acceptable ways for lesson barns to handle horses that need to retire due to age or soundness issues. Caring for retired horses is extremely expensive.
From a business perspective, once a week or twice a week lesson students generate very little revenue compared to people who own or lease horses. Lesson programs are really only useful on a small scale as an entry point for new clients. Dealing with each individual client is time consuming for a trainer, and a busy lesson program involves a lot of different individual students who might be very time consuming in terms of scheduling, messaging, questions, billing, etc. but who might only be generating four lessons worth of revenue per month.
I agree that the limited availability of lessons on respectable school horses is problematic for the horse industry and makes the sport less accessible.