Sure, searching with an external search engine is an option, but YouTube and TikTok also just bring videos to you. In other words, they offer passive discovery you just don’t get with a search engine.
It’s fine if that’s not what you want, but people do love it and it keeps them going back to YouTube and TikTok.
If I want to find videos on a specific topic, I use a search engine. DuckDuckGo yields much better results than YouTube, even when the results are hosted on YouTube itself.