Because it costs a massive amount to get standards with this technical quality. They go through meetings, have a decent amount of staff to run, organize, have conference costs (locations...), take years to get done.

Someone has to pay for this. Making companies (and often, many of the individual members do this out of their own pocket) pay it all means worse standards, as some people stop going.

Sharing the cost to make the standard makes it a better mix of getting good standards and having low costs for final users.