Why is there no implicit conversion from std::string_view to std::string?

The problem is that std::string_view -> std::string makes a copy of the underlying memory, complete with heap allocation, whereas the implicit std::string -> std::string_view does not. If you've bothered to use a std::string_view in the first place then you obviously care about copies, so you don't want one to happen implicitly.

Consider this example:

void foo1(const std::string& x)
{
    foo2(x);
}
void foo2(std::string_view x)
{
    foo3(x);
}
void foo3(const std::string& x)
{
    // Use x...
}

The function foo2 could've used a const std::string& parameter, but used a std::string_view so that it is more efficient if you pass in a string that isn't a std::string; no surprises there. But it's less efficient than if you'd just given it a const std::string& parameter!

• When foo2 is called with a std::string argument (e.g. by foo1): When foo2 calls foo3, it creates a copy of the string. If it had a const std::string& argument, it could've used the object it already had.
• When foo2 is called with a const char* argument: A std::string copy has to be made sooner or later; with a const std::string& parameter it gets made earlier, but overall there's exactly one copy either way.

Now imagine foo2 calls multiple functions like foo3, or calls foo3 in a loop; it's making exactly the same std::string object over and over. You'd want the compiler to notify you about this.

Because expensive implicit conversions are undesirable...

You only listed one implicit conversion out of your entire set of examples: const char* -> std::string; and you're asking for another one. In all of the other 'OK'-marked functions - allocations of memory are obvious/explicit:

• Assignments: When you assign anything to an owning object with variable storage size, it is understood that an allocation may be necessary. (Unless it's a move-assignment, but never mind that.)
• Append I: when you append to an owning object with variable storage size, it is even more obvious it will need to allocate to accommodate the additional data. (It might have enough reserved space, but nobody guarantees this about strings.)
• Concatenations: In all three cases, memory is allocated for the result only, not for any intermediate object. Allocation for the result is obviously necessary, since the operands of the concatenation remain intact (and can't be assumed to hold the result anyway).

Implicit conversions in general have benefits and detriments. C++ is actually mostly stingy with these, and inherits most implicit conversions from C. But - const char* to std::string is an exception. As @ArthurTacca notes, it allocates memory. Now, the C++ core guidelines say:

C.164: Avoid implicit conversion operators

Reason:
Implicit conversions can be essential (e.g., double to int) but often cause surprises (e.g., String to C-style string).

and this is doubly the case when the unintended conversion performs expensive operations like allocation, which have side effects, and may make system calls.

PS - std::string_view has quite a few gotchas; see this CppCon 2018 talk by Victor Ciura:

Enough string_view to hang ourselves with

So remember it's not some kind of universal panacea; it's another class that you need to use with care, not carelessness.

... an explicit constructor is sufficient.

There actually does exist an explicit constructor of std::string from std::string_view. Use it - by passing std::string{my_string_view} to a function taking a string.