You don’t have to ask the user, there is a media rule for querying whether the device is currently using coarse or fine pointer input[0] (though, of course, it relies on the OS not lying, which is not a given).
Some websites probably do have settings around this, but that gets to a point someone else mentioned: you would have to basically design, build, test and maintain two UIs. Except now with the kicker that one of those layouts is only used by the 5% of your userbase that both knows that the option is available and chooses to take you up on it.
