I couldn't find an explanation to why this is so. Is there something silly I'm missing?
This is to do with the original request for comment (sort of a pseudo standard for internet things) which was based on the limitations of the original technology. We have gotten to the point that by and large the underlying technology of DNS would support more than 127 zonefile recordsets and more than 255 characters however the limitations of the DNS protocol itself (which was based on the RFC) mean that these limitations are still in place.
Additionally even though technology marches on there are still places, especially larger or more well established places online (think major tier 1 and 2 networks) where they are using older generation hardware and DNS software to do the job where these limits are still an issue.