The problem is that most web pages and programs don't actually do very much when they're in the background, and many (most?) of the software that does grind away for long periods of time without user interaction (compilers, ray tracers, video encoding etc.) are already able to make good use of many cores. But users who fit that profile are a niche market.

What the vast majority of users need in terms of speed is acute performance. A dedicated core for every webpage is not very useful because users don't load up a bunch of tabs at the same time and then flip between them many times per second, interacting with each one. Instead CPU usage tends to happen in short bursts directly following user interaction, so even if every tab has its own core, you won't usually see very much contention for CPU between different tabs. The same goes for most native programs. Users just don't multitask fast enough to make separate cores relevant for most programs.