Video distribution is expensive and technically complicated.
Say I've got a video channel and most of my content is "talking head" style. Encoding 1080p using in a widely compatible h.264 configuration (Main profile level 3) I could easily get good quality output at a data rate between 1-2Mb/s, let's say 1.5Mb/s. That's about 11MB per minute of video for storage purposes (for that single config).
In order to serve those videos to streaming clients I'll need 1.5Mb/s of bandwidth for each client. Any lower of a transfer rate and the playback will stall. Faster data rates can allow for a larger buffer but the file's data rate is your transfer minimum to actually play the content as it downloads.
So back to our video channel. Self hosting on my residential connection means I'm limited by my shitty upstream bandwidth and throttling from my ISP if not outright T&C violations. My home connection I could serve maybe a half dozen clients simultaneously. So long as my content is unpopular that might be ok. Bubbling up on an aggregator or just making an interesting video would DDoS me and no one would get to see it. It I want to spend ~$60 a year on cloud hosting I could serve a lot more connections. Then I'm out $60 a year or more if I want to make a longer history of my content available, remember 11MB a minute.
If I want to have better compatibility and broader reach I would need to encode a couple more versions of my videos at the cost of storage and my time.
So self hosting either costs me money or severely limits the availability of my content. P2P distribution can help a little but it has its own issues. Even an aggregator offering paid hosting doesn't really solve the issue because I'm still out some money. Were I to make any money from advertising or patronage that hosting cost would come right off the top.
The situation is even more difficult if you consider mobile access which is a huge portion of online video access. For a lot of reasons self hosting from mobile devices isn't practical. P2P from a mobile device is likewise impractical. So for anyone with a mobile device as their primary computing device, a non-trivial number of people, they would have to pay for some third party hosting without a meaningful self hosting option.
Demand from mobile devices also significantly increases your need for multiple encodings of a video. Mobile streaming needs to respond to much more drastic traffic conditions than your laptop on your home WiFi. This increases storage requirements meaning less historic content or increased storage costs.
TrainVids.com doesn't get many hits so for now they rely on the source' perfomance, but CatVids.com gets millions of hits and they long ago invested in a redistribution layer. Meanwhile ContentCreatorMatt pays $5/m on his source hosting with burst protection should a video go viral. Hosting can afford this because they have thousands of creators who never get a hit video. While CreatorTom only submits to large platforms with redistribution so he doesn't have to pay for traffic.
I wouldn't use P2P nor expect people to host the videos at home because that would never work for the reasons you outline. But think of live streaming and how that distribution works and that is just one possible method an aggregator could choose to increase performance of slow sources.
The idea is to break up the video platform monoculture and make the space competitive again, so that there will be some variance in performance to end users depending on the aggregator they visit isn't an issue.
There probably are ways to keep a catbirds.com running but as the difficulty increases the likelihood of the site's maintainers keeping with it decreases significantly. If the site tanks all of its hoisted content goes with it. Ask all the people that had their content wiped from MP3.com how that feels.
So from a content creator's point of view, why hitch their wagon to catvids.com? It costs them money and isn't guaranteed to be around in a year. They're also far less likely to make any money no matter how much traffic aggregators send them. Sticking with YouTube is way less of a hassle and gives them a far better chance to make money if nothing else to break even to keep making content.
The aggregators don't own or host the content in the system I am thinking of, they just consume and redistribute their sources. So if catvids.com goes away the creator's source is still online. It's also probably syndicated to a bunch of other cat video sites simultaneously from that one source. Thinking through it more, it's pretty much an RSS feed but for videos. In fact I would probably just use the RSS protocol if I were ever to try and get something like this off the ground. Have an open source aggregator template so it's easy to start one, and then monetize it by starting the first video source hosting as a service company.
I don't think it could ever take off though, the "Next YouTube" will be another massive corporate, closed platform with deep pockets or venture capital.
You're right though of course, which is why we have a YouTube monopoly in the first place.