First off, most processors are not BE. Arm and X86 are LE and cover "most processors".

Second, processors like the Motorola 68000 were 32-bit, so there was no need to read just the first byte. Refer to my point about instruction fetch. Like I said, if you have an ISA that came from byte-sized opcodes, and then expanded later, you need to fetch bytes first. However, if your opcodes are 32-bit, then the code is bigger. (In the old days, CISC code size was a selling point compared to RISC, this opcode compression was one of the reasons.)

This is what you learn in first-year computer architecture. However the world has changed a lot since the 80's. Single bytes aren't fetched in x86, 256-byte cachelines are, and the decode happens on that.