Uninitialized memory is not a problem (the OS is never going to give a program memory that has data in it from another program). The problem is memory that you allocated in the past, have freed, but hasn't been returned to the OS[0]. It might have key material or other sensitive data in it[1]. Or it might just have random garbage in it that could be misinterpreted by the code that's about to use it, if it hasn't been initialized to a known state.
For some uses, you do genuinely need (specifically) zeroed-out memory before you start to use it, and that's where calloc() is truly useful. But that need not have anything to do with security.
[0] The allocator will often hold onto memory that has been freed in order to quickly service future requests for new allocations, without needing a context switch into kernel space.
[1] Granted, the correct way to handle that is to zero it out before freeing it, in a way that the compiler won't optimize out.
> The problem is memory that you allocated in the past, have freed, but hasn't been returned to the OS[0].
There are at least two different ways in which memory might be semantically "uninitialized":
1. The memory was provided by the OS. On modern desktop and mobile OSes, this memory will normally be zeroed automatically.
2. The memory was provided by the language's allocator. This may contain a mix of data used by previous allocations and memory that has never been touched (perhaps because previous allocations reserved it as end-of-array "capacity" that never got used). From the perspective of a language like Rust, this memory is considered uninitialized, and safe code should never be able to read it without first setting it.
In ancient C code, it makes a fair bit of sense to preemptively calloc everything. Or better, to wrap the allocator with one that zeroes on free. Though even there, you need to be careful not to expose recycled heap block headers in the middle of newly allocated objects.
My opinion for the last 30+ years has been that C is unfit for purpose, and that using it almost inevitably introduces large numbers of dire security holes. But until the last 10-15 years, there hasn't been any seriously viable alternatives.
You can avoid this overhead if you use a language that forbids reading from uninitialized memory, but C is not that language.