Why these ad formats?

Because they work. Open the Meta Ads Library and look at big brands: many rely heavily on animation, since it is visually rich, stops the scroll, and lets you tell a story that keeps viewers watching. SaaS and other digital products lean on motion graphics in their marketing. Slideshows are excellent for content-style top-of-funnel ads because at first glance they do not look like ads at all. Video banners are a cheap way to run testimonials or short product descriptions in the feed.