Do you not worry that it turns off beginners ?
Do you not think that the wheel sizes on cars would put off a new driver learning their skills?
Low profile tyres cost so much more, and does the driver really have to replace those tyres with "like sizes or can they use another type or size?
But in reality when buying a car for the first time do they ever think about buying the most economical to run.. NO its the one that the cheapest to insure as thats the one big impact that new drivers actually pay for.
So, why don't they make a standard size wood turning lathe with standard headstock threads so all the wood chucks manufacturers can make a standard size for the new wood turner..No they provide thread adapters for that, a cost that a newcomer would have to buy to enable them to use a specific chucks.
Does that put people off taking up wood turning? Does it concern anyone? I don't believe so. All they want to know is can they buy parts for their lathe and can they get it to do what they want in life so they can enjoy their new found skills.
The original Question of the thread was a concern for the OP and I guess that when getting in to pen turning now realises that there is an additional cost to buy bushes to suit every different pen.
That is a learning curve for all of life skills that we take up.
If I wanted to learn to fly an aeroplane I could just go out and pay to have lessons and obtain a licence.. In those lessons there is nothing to say how much a plane would cost to buy, run or even disclose the landing costs of the airports / strips you will need to use.
So in reality your questions of "standard" sized bushes making universal fittings may be an option but its not the biggest problem any new pen turner would face.
This conversation could go on with analogy for everything, why not 110v rather than 220v? why not all convert to Euro plugs rather than a UK plug. Why not all move to the American style electrical plugs. It would help millions of travelers around the world daily rather than a few penturners having a standard size of bushes..