I believe the most common cause of cookie deletion is hitting the browser cookie limit, and old cookies rolling off. The sheer number of JavaScript tags on webs

I believe the most common cause of cookie deletion is hitting the browser cookie limit, and old cookies rolling off. The sheer number of JavaScript tags on websites (I've seen a list of over 600 third party trackers), multiplied by the number of sites you visit for those trackers that create per domain cookies (for example google analytics), then add in the multiple unintentional cookies created by programming frameworks on each domain, and the limit gets hit pretty fast.