Because some of us don't want to use Chrome? How is vendor lock-in at the browser level any better than at the OS or device level?
Small software companies have to make platform decisions. They chose the most popular browser (and notably most secure browser), allowing the app to work on all OSes.
Chrome is a good, modern browser but this constant pushing by google (telling me to download a "better browser" when I'm visiting their site in Firefox) as well as every lazy web developer annoys me (seriously, at least consider if you should test basic functionality in all major browsers even if you aren't directly paid for it) .
Still: wish less companies would be fine with shipping web apps that break in one or more of modern browsers.
I think both aspects of this statement are definitely not clear - certainly not notably (reference?), and comparisons based on some searching seem to go one of numerous ways.
I would be happy to see a security expert's view on this though.
See also this link on Chrome devs essentially telling users that upgrading their kernel from 3.16 (not even > 7 months old at the time): https://news.ycombinator.com/item?id=9164251. Thankfully better sense prevailed and it was resolved fast. So even in terms of OS support it is not unambiguously better than others.