Normally you want to reserve the "unused" opcodes in order to assign them new semantics (new instructions) later in an updated CPU model. To discourage programs from using those yet unassigned opcodes, CPUs usually trap if they encounter them (i.e. raise an "illegal instruction" interrupt).

The 6502 approach of just executing some random (but deterministic) logic leads to a situation where the "illegal" opcodes cannot be expected to be unused (programs actually use them), and where assigning them new semantics effectively breaks compatibility with existing programs. That’s why CPUs usually invest in the extra logic necessary to give those opcodes undesirable behavior (i.e. traps).