This is done in an effort to comply with what is commonly called "cookie law" (GDPR + ePrivacy directive).

Those texts don't say anything about cookie banners, though, only that users should be informed and have the possibility to make a choice. How that information and ability to make a choice are presented to the user is left to the site owner to decide.

For site owners, cookie banners are an easy way to comply without having to DESIGN that compliance into their site, which can be interesting but time consuming and expensive.

In theory, it should be possible to prompt the user for consent before storing something on their device but that would only work for stuff site owners have control over.

In practice, "cookies" are rarely used voluntarily by the site owners themselves but they are used a lot by the crap they add to their sites for tracking and so on via tag managers. And for that there is no better solution than a blanket cookie banner.