People who are 65 or older usually see doctors more reliably than younger people.
And it takes a lot of knowledge in order to correctly administer the correct booster at the right time to people...this isn't something that can be done by a Walmart or CVS doctor...there's a huge potential for harm and eliminating any protection against Covid.
Also the other pertinent fact is that there are several variant boosters in process of getting approval. (Different than the original) these boosters will provide better protection against the variants than the original will. Again crossing companies and types will likely do more harm than good.
This isn't exactly as simple as the information we are getting fed...there's no "all or nothing" about any of this stuff. It's complicated with varying degrees of risk/reward that have to be carefully measured.
What do you do with a 70 year old guy with a J&J vaccine history (he thinks but isn't sure) when giving him a Moderna booster will destroy whatever protection he has?