A "regular file" might reside in multiple locations on disk for redundancy, or might have a checksum that needs to be maintained alongside it for integrity. Or, as you say, its contents might not reside in contiguous sectors - or you might be writing to a hole in a sparse file. There's a lot of "magic" that could go on behind the scenes when operating on "regular files", depending on what filesystem you're using with what options. Directly operating on the block device makes it easier to reason about the performance guarantees, since your reads and writes map more cleanly to the underlying SCSI/ATA/NVME commands issued.

If you understand your workload and the hardware well enough to understand how doing direct I/O on a file will help - then you’re going to generally do better against a direct block device because there are fewer intermediate layers doing the wrong optimizations or otherwise messing you up. From a pure performance perspective anyway. Extents are one part of the issue, flushes to disk (and how/when they happen), caching, etc.

Doesn’t mean it isn’t easier to deal with as a file from an administration perspective (and you can do snapshots, or whatever!), but Lvm can do that too for a block device, and many other things.