I think you underestimate how many i3 BMW have sold, they built almost 100 000 from 2013 to 2017[0]. McLaren on the other hand celebrated 15 000 cars total in may this year[1]. BMW also have a smaller budget for each tub so it's pretty impressive that they have a production car with a carbon fiber tub.

[0] https://en.wikipedia.org/wiki/BMW_i3#Marketing_and_sales [1] https://www.forbes.com/sites/nargessbanks/2018/05/29/mclaren...