They already did that, and web "developers" did not do the work. The period in which Google incentivized faster page loads was the period of some of the worst page bloat growth.
In some areas people understand care about such things, in those areas we did and still do see some optimisation effort. Far from everywhere of course, even within that subset, but I'd wager there was improvement overall (or at least things got worse less than they otherwise would).
In other areas though the available audience either doesn't understand or doesn't care (because their connection and CPU are fast enough to cope for instance, and they aren't on an expensive metered link) or both. I say "available audience" because for these sites the "target audience" is every-human-alive-several-times-over-good-god-we-need-more-ad-impressions-and-clicks - the likes of buzzfeed and the other click-bait filled "news" and "entertainment" sites where four sentences of content can be stretched over sixteen pages of large, slow loading, auto-playing, obnoxious adverts. The people who care about load time (and/or relevant technical or privacy matters) simply don't follow those links. The people who do follow the links aren't worth making page load optimisations for because they simply won't care or notice, so from the site's PoV the time is better spent on adverts-hung-onto-into-each-morsel-of-content optimisations instead. And that was when Google ranking was the main thing that mattered, more recently the operators of such sites are more interested in distribution via social media so Google ranking you lower for a slow load is less of a concern, or if it is a concern there are tricks (not all of which Google can easily filter for - it is an on-going war of attrition) to show Google different versions of the content that does load quickly.
I would argue that they didn't do that, or at least not enough. I obviously have no insight to how they sort, but IME they aren't putting small and fast sites at the top of the list. Those are usually 3 or 4 pages back, and the first results are usually the slow and bloated ones.
