Chrome and Firefox are as secure as Safari, if not more, banning them is a commercial choice not a technical one.
iOS exploits still exist, there's no real advantage in Apple sandboxing apps, they are routinely leaking users data and being exploited as well.
OTOH Apple refusing to implement certain web standards is proof that they cannot guarantee a safe implementation, which is a reason more to allow better browsers on their platform.
This Apple native crud they force every app to use (up to the whole browser engine, like in the IE days!) is truly the ActiveX of our times. Only you can't even get rid of it.
You can exploit in both native and browser contexts. Most jailbreaks nowadays are assisted by a native application that you dev-sign to deliberately pwn yourself with. In the past we had websites that you could use to jailbreak with. Both are sandboxed environments with significant attackable surface area, so one is not necessarily more trustworthy than the other purely on measures of exploitability.
A lot of viruses (and jailbreaks on iOS amongst others) are distributed via this browser / sandbox; it's only secure in theory and it took decades to get to that point.
Sure (before the Rust evangelists swoop in), part of that was due to using unsafe languages; part was due to extension frameworks that had too much power (ActiveX, which was even used to update your operating system, I can't fathom why they thought that was a good idea). But it'll take many more years of zero incidents, jailbreaks, etc before I'd trust the browser over Apple's app sandboxing and app review and distribution approach.