> for example having a small widget in the footer or elsewhere on my site

And next thing you know, there'd be people complaining about "dark patterns" where the small widget is designed to be very easy to overlook and (more importantly) someone may inadvertently use the site briefly before noticing that they can turn off tracking. Whoops, you just got tracked.

A modal makes it obvious what's happening on the site and gives you a chance to opt-out before proceeding. There really is no better way to do it that's going keep everyone happy. People just don't want to admit that the only way to properly implement this feature is to annoy people.