If browsers were slightly better at asking users, maybe we'd all have our three favourite fonts and background-, text- and link-colours instead of what someone else prefers we stare at all day.
What if I want websites to have different fonts because it gives them unique voices? Also, what about websites that use OpenType features only supported by some fonts (small caps, math, variable axes, etc.)?
Some of my favourite fonts I only discovered because I first visited a site that included it in their design.