No doubt many of these videos took many attempts (often by children) to get the animal to do something 'funny' and yet painful, sometimes beyond the threshold of what is considered animal abuse, at least in my country. All for a few likes.
Like saying "thousands of 'funny' TikTok videos and iMessage gifs", it seems weird to group TikTok and WhatsApp this way.
WhatsApp's one and only feature is private direct messaging. Basically equivalent to iMessage, SMS, eMail, etc.
edit: Did you mean that this content is being spread maliciously by spammers using WhatsApp?
As an example; the official Valorant Discord server has 200,000 people online right now. Not total; online, actively receiving messages. Some of these private WhatsApps, Discords, etc have tens of thousands of active members, and the content shared goes far, far beyond whatever the title of the channel indicates. The companies behind these isolated social network islands may do some light moderation on the overall server/channel, like not allowing terrorist groups to communicate, but very very rarely do they moderate any deeper; that's left up to the owners of the channel/server, and if they're complicit, it'll never be cleaned up.
Of course, WhatsApp, Discord, Telegram, etc, they don't do discovery. They just let third parties handle that [1]. They get a ton of plausible deniability, which is even further empowered by the push for E2EE.
After AFV started increasing this I just had to stop watching it. YouTube of course has its share of Awful, just like everything else on the Internet.