Once a system gets gamed en masse, anti-gaming measures dominate, and ruin, the whole process. See also: ads vs. ad blockers, and the fact that you can't run a fucking email server of your own anymore without jumping through security hoops.
How far back are you referring to? I'm definitely not the oldest person around here, but most of my interviews since graduating college were more than casual conversations.
The exceptions were primarily where I was coming in via a referral or they were hiring me for something that was already proven and verifiable, like open source contributions.
There were some companies like Microsoft who were starting to become known for being more intense, but I never came across that.
It wasn't until the mid-aughts that I started to see brain teasers, "real" whiteboarding, and other aspects that started to bleed into standard interviews.
Perhaps the turning point was Jim McCarthy's "beware of a guy in a room", which he meant both figuratively and literally, leading to a transition away from solo or shared offices or even cubicles for software engineers and toward open-plan arrangements. I think that his wisdom led first Microsoft, then the rest of the industry, to treat software engineers not as the darling whiz kids they were in the past, but more a cohort to be closely watched and controlled, as the company is risking a lot on them.
interviews at twitter back in 2010 were 6 hours and that was true even for me when we had an agreement to acquire my company pending the success of those interviews. at the time they had a lot of ex-googlers and they were setup similarly.
In 30 years of interviewing on both sides, as candidate and as hiring manager, I’ve never seen that myself. I don’t doubt it’s happened. I can’t speak to what’s normal everywhere. But I can say for sure that either a half day or full day of formal interviews (usually depending on both team and company size) has been absolutely standard in my experience, both in Silicon Valley and elsewhere, for decades.
On top of that, all of my jobs I’ve ever had, even when I was a shoe-in because I got the interviews via networking, they all involved multiple days worth of email and phone calls and resume prep and benefits negotiations, and (later in my career) sometimes legal negotiations like non-compete exclusions.
I’ve had lots of casual conversations during the process, but I’ve never been hired on a casual conversation alone, and never seen someone else hired that way.