This is by design.
[class.compare.default] (emphasis mine)
3 If the class definition does not explicitly declare an
==
operator function, but declares a defaulted three-way comparison
operator function, an==operator function is declared implicitly
with the same access as the three-way comparison operator function.
The implicitly-declared==operator for a class X is an inline
member and is defined as defaulted in the definition of X.
Only a defaulted <=> allows a synthesized == to exist. The rationale is that classes like std::vector should not use a non-defaulted <=> for equality tests. Using <=> for == is not the most efficient way to compare vectors. <=> must give the exact ordering, whereas == may bail early by comparing sizes first.
If a class does something special in its three-way comparison, it will likely need to do something special in its ==. Thus, instead of generating a potentially non-sensible default, the language leaves it up to the programmer.