To continue on what
@vincent above said, recommending something is more than just about knowing what someone likes, it's about finding the content that person likes as well. Tiktok has better content BECAUSE they're their algorithm is able to promote that better content while other platforms let them languish in obscurity.
Yes, there are differences between the approaches used by Reels, Shorts, and Tik Tok, but what I'm saying is, it would
not be difficult for engineers in Google or Facebook to replicate the Tik Tok algorithm. The algorithm is not the "secret sauce" that people treat it as. You can easily tune a recommender to promote high quality content through boosting content that has high engagement, likes, and which come from trusted sources, and with many content creators releasing their content cross-platform, you could even run comparison studies to validate that you've replicated their algorithm.
So why haven't they done this? The reason is the community & product lock-in. Reels is an extension of Instagram, and Instagram is already locked in as a platform for thirsty men. If you've ever gone on Instagram, you know that if you set your profile to male, you'll be flooded with half-naked women
by default. People know this, and it's why they (as in, people who aren't thirsty men) steer clear from Instagram. If they switched to a algorithm that promotes viral content, it'll just be promoting more half-naked women because that's what their community likes.
Shorts is similar; they're an extension of Youtube and dominated by Youtube personalities. It's one reason Shorts is not as popular - if someone's already on Youtube then Shorts is just a side piece for them. It's again a product & community problem. Google is constantly worried about Shorts and Youtube cannibalizing each other's engagement. Shorts will forever live in Youtube's shadow because Google will never shake the boat with Youtube.
Tik Tok is free from such constraints. Tik Tok was built from the ground up as a short form video platform and not an extension of another platform/community. Byte Dance built the Tik Tok community to be far more
young, female, and trendy compared to its competitors.
You can't replace that just with an algorithm. These people have no alternative. That's why they've been actively rebelling.