Except that the market isn't there for it, so third parties aren't doing it effectively, yet. Since that is the case, we have to focus efforts on getting one easy-to-use, easy to obtain and install, and well-maintained GUI out there.
I'd say the same thing about exchanges or anything else if no one were providing the service (in fact I sort of did that with the OTC trading thread in the very early days of Monero).
With greater adoption and scale (including liquidity and market cap), more third parties will be able to effectively provide these products, and they will likely do it better because they will be able to specialize on different feature tradeoffs and particular market segments (including grandmas), but until then we have to fill the gap with something reasonably suitable for a generic "user" if we want to get anywhere.
But such a solution already exists. MyMonero is already much simpler than downloading the core software and running a node, even with a bundled GUI. Why would grandma (or any "generic user")
need the core GUI over MyMonero? Who does it really serve?
Security.