Not nobody! This proposal discusses some of the tradeoffs with exposing the size given a pointer, while proposing a different way to give the true buffer size:

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3899.pdf

Many fast allocators do not store the size next to the allocation, and may not store the requested size at all. This has the advantage that freeing a large allocation does not fault in cold pages with a write, as well as reducing allocator memory overhead.