The cost.

Closed source and open source developers use the same $300-3,000 laptops / desktops. Everybody can afford them.

Training a large model in a reasonable time costs much more. According to https://lambdalabs.com/blog/demystifying-gpt-3/ the cost of training GPT-3 was $4.6 million. Multiply it by the number of trial and errors.

Of course we can't expect that something that costs tens or hundreds of millions will be given away for free or to be able to rebuild it without some collective training effort that distributes the cost on at least thousands of volunteers.