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?
Grandma would probably use MyMonero over the core GUI (though maybe not for longer-term storage of value). That is a good example of third party solutions starting to specialize.
By "generic" user I didn't mean grandma but a wide base of all sorts of users without specialization, and that's where MyMonero fails to hold up. That includes people who see the primary use of crypto today and for the indefinite future as a store of value, medium- to long-term speculative investment, or perhaps as a medium of exchange that is somewhere between reasonably and very private. In all of those cases many won't want to use a web wallet.