Do Apple laptops use FileVault encryption by default? Has Apple ever rolled out MacBooks with FileVault enabled by default? If yes, which version(s)? If no, is there a reason why they wouldn't?
Apple has not, to my knowledge and research, ever released a machine with encryption turned on by default.

Even with APFS that natively supports encryption* (meaning it's baked into the cake and not just another layer on top), encryption has three options:


no encryption
single key encryption
multi key encryption


This is something the user must select.  Why Apple chose to go this way is something that only Apple knows, but it's evident that they wanted to give the option to the end user (or at least to the people deploying the Macs to the end users).


* 9to5Mac.com:  Apple File System (APFS) announced for 2017, scales ‘from Apple Watch to Mac Pro’ and focuses on encryption