Because it’s way more convenient. Next time you go to open your front door, take out your phone, start a timer for 30s, and then put the key in the door once the timer expires. Once you’re used to something being near-instant, there is no going back. Waiting is frustrating.
Personally I haven’t used hibernate in over a decade and my machines typically run 6 months until I have to reboot for updates. If I had to wait ~10 seconds every time I wanted to google something, I wouldn’t use my PC nearly as often as I do today. Maybe that’s why mobile is dominant in so many categories like web search.
You're right that this is why I use my phone to do quick searches so often- it's on and searching within seconds. Even faster to use my voice to search instead of typing.
On my Framework (running Windows) I have it set up to hibernate when the lid closes to preserve battery. It takes about 10 seconds to get back into the desktop when I open the lid again. It's not like I'm opening/closing the lid dozens of times a day; I open it, work for a few hours, then close it. It's not like a phone where I power it on for a few seconds every hour or so to check notifications.