Others have talked about the size of the market (in terms of the number of customers) and the high entry costs, and those posts are correct and are the reason why no one starts a new passenger aircraft company today
However, it’s only half the story. There USED to be many companies making passenger aircraft. Over time they have merged into Boeing (mostly) so in the 60s and 70s when the market was just getting going there were many, but over time the successful one(s) bought the smaller ones and kept getting bigger
But at one time there was McDonnell, and Douglas who merged into McDonnell Douglas. Eventually, Boeing then bought MD. There were others, of course, but over time all of the American companies have ended up as part of Boeing as it got bigger and had the sole ability to buy up all the little guys
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