Because for any individual seller there’s a heavy short/medium term advantage to using the marketplace in the form of dramatically increased reach and simplified logistics.

You got two answers that are telling you this is good and normal, but I have a third one:

They're the only game in town at this point.

You could ask why people start businesses that rely on buying stuff for x and then selling it for y when that business model has already been fully exploited?

There is a supermarket chain called Aldi who's entire business seems to rely upon copying branded products yet they are lauded for offering great value?

Because there's not another marketplace where you can, with some effort and very little actual innovation, turn a $5,000 investment into a six (and sometimes seven) figure a year revenue stream.