It definitely would be great if you could use your preferred tools and build from the commandline, or if Apple could ease the process of self-signing for local deployment.

However, the tension between developer and user friendliness as noted elsewhere in this discussion applies here as well. Presumably a part of the fees that Apple collects from developers covers the cost of auditing extensions for security, and identifying developers which provides a disincentive for user-hostile activity.

Meanwhile, Chrome extensions have been a vector for adware and malware–often because bad actors will buy popular but unmaintained extensions and users will get an updated version that is user-hostile in some way. One could argue that Apple should fund the auditing process out of other revenue, and that it is short-sighted of them to try to make the developer ecosystem self-funding on the backs of deveopers of free extensions, but either way, I've felt a lot safer recommending Safari to my less technical friends and family than Chrome for this reason alone.