In the past, it was extremely expensive to host users' photos. Forums generally only allowed users to upload a single avatar, and any images you wanted to embed in a forum post needed to be hosted in a separated website that specialized in image hosting and image-sharing and that allowed embedding images uploaded to that site to be embedded in third-party forums. This is the reason why Imgur exists, for example. It was, originally, Reddit's image-hosting service.
As storage became cheaper and Internet speeds faster, platforms started allowing users to upload more photos than the average user ever would, but video in any format was still prohibitively expensive. Only Youtube, with Google's vast infrastructure, would host videos of anybody on the Internet for free. Its main competitor, Vimeo, had a very limited free tier and the uploader had to pay a subscription to upload more videos.
Nowadays that is still true. In fact, it will probably always be true. Allowing anybody on the Internet to upload videos to your website leads to all sorts of problems, specially with so many people uploading 10 hour videos to Youtube that are just a small 10 second clip looped for 10 hours. This is funny when you're hosting it on someone else's website, but it's not funny when random people are putting that sort of content your servers that you have to pay the bill for.
Fortunately, platforms figured a trick that lets them maximize profits while minimizing the costs of hosting video: the short video format. Shorter videos require less resources to host, transmit, and moderate, and they are also easier to monetize with ads. You can just put a video ad between every 3 short videos and most people won't even be able to tell they just scrolled through an ad every time it happens.
Today, even Imgur has become a TikTok clone.