There's one important factor you're missing: for lack of a better word - "staffability". Just about every programming language has also a web framework impleme

There's one important factor you're missing: for lack of a better word - "staffability". Just about every programming language has also a web framework implemented in it, so if you want to create a full-stack web app, the range of options is just dazzling, which results in a highly fragmented ecosystem. Meanwhile on the front-end there's essentially one language(with different flavours) and 3-4 leading frameworks, so once a developer has these covered, they can be hired to do anything in this part of the stack. The overall result is that it's easier to scale work if you offload it to the front-end, because by the time you found your <insert highly specific language/framework combination>, your front-end developers would have done most of the work twice over. There are many more reasons why you would want a SPA instead of a full-stack app, but this is one of the strongest.