Back through the 90s and 00s, I think that ~3 years was the norm, at least for the part of the population that even owned computers.
But since then, there are fewer major improvements coming that fast. GPUs improve, but most people aren't hardcore gamers for whome that matters, and these can mostly be upgraded independent of the computer itself instead.
Other than that, if you're mostly just using your web browser, and maybe doing some spreadsheets and desktop-based email, what's the point? Even if you're doing more demanding stuff like Photoshop, we're at a point where the app's demands aren't growing fast either.
But since then, there are fewer major improvements coming that fast. GPUs improve, but most people aren't hardcore gamers for whome that matters, and these can mostly be upgraded independent of the computer itself instead.
Other than that, if you're mostly just using your web browser, and maybe doing some spreadsheets and desktop-based email, what's the point? Even if you're doing more demanding stuff like Photoshop, we're at a point where the app's demands aren't growing fast either.