Because it’s an extremely difficult thing to do, and requires a long history of manufacturing to be competitive. Designing and building a large airliner that is safe take vast resources, especially if you’ve never done it before. We’re talking years of R&D. The chances that you’d be able to do it better than an established competitor and still turn a profit are low.
Both Boeing and Airbus started when things were simpler, and have built up to this point. Besides, the market is only so large. Anyone trying to get into it would have to take sales from them. The only way to do that is to offer something better or cheaper.
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