> given how bloated modern websites are. It’s easer to imagine a cheap phone with a set of optimised native apps

The age old truism in tech is that it's easier to hyper-optimize a single chokepoint (i.e. the platform) than trust distributed actors to each optimize their own thing (i.e. app developers).

Because optimization takes money. Sometimes lots. And distributed non-platform actors have their own priorities like features.

That's why you get enormous success stories like x86, TCP/IP, HTTPx, and Javascript engines providing increased performance, but the "developers as a whole self-optimize" dream remains a perpetual mirage.

(Outside of gaming console hardware, and even there arguably true before middleware)