Today's browsers tend to be huge memory hogs too. Software's attitude of "there's always more memory" is coming back to bite them as prices of ram increase.
IMHO, browsers might prioritize execution speed somewhat more than memory. There is the Pareto tradeoff principle where it's unlikely to optimize all the parameters - if you optimize one, you are likely to sacrifice others. Also more memory consumption (unlike CPU) doesn't decrease power efficiency that much, so, more memory might even help with that by reducing CPU usage with caching.