Why do MIPS subroutines return by jr while x86 return by using ret?
Why isn't there any ret instruction in MIPS?
Alternatively, why doesn't x86 use jr?
In x86, the call instruction puts the return address in memory on the stack. In MIPS, the call instruction (jal) puts the return address into a register.
In x86, the ret instruction removes the return address from the stack and branches to it. The MIPS philosophy doesn't combine separate steps into a single instruction. This simplifies the instruction set considerably. Removing a value from the stack and performing a branch are separate operations, requiring separate instructions. In the particular case of call/return, it also has the advantage that in a leaf function there is no need to write the return address to memory at all.
X86 has an equivalent of the jr instruction, jmp r/m32, with the destination specified as a register. This instruction is not usually used for function return because then a separate instruction would be needed to pop the return address from the stack into a register (but I have written code that does exactly that, in special circumstances).