SIP-45 - Clause Interleaving justifies the lack of support of LHS of extension methods as follows:

The left hand side of extension methods remains unchanged, since they only have one explicit term clause, and since the type parameters are very rarely passed explicitly, it is not as necessary to have multiple type clauses there.

The following example demonstrates how such support would indeed be beneficial to the language.

trait Foo:
 type Type 
// Ok
def id(using foo: Foo)[T <: foo.Type](t: T): T = t
// Not ok
extension (using foo: Foo)[T <: foo.Type](t: T) def id: T = t
 ^
 '(' expected, but '[' found