Linux has over 25 million lines of code (as of 2018 [1]). Would an audit be straightforward?
[1] https://www.phoronix.com/scan.php?page=news_item&px=Linux-Ke...
[1] https://www.phoronix.com/scan.php?page=news_item&px=Linux-Ke...
Compare this to something like, for example, TikTok, where the code is completely closed. The only kind of auditing one may do is looking at network traffic, and maybe some disassembly/hex-editing.
I'm not saying that everything needs to be OSS but it is nice to have options.
If you don't have code you can't audit code!
Tiktok is far smaller, and has a smaller attack surface. If there was money in it, people would audit Tiktok all day.