Just because you don't see the problem, it doesn't mean it's not there. However you want to put it, you can't add (or remove) instructions at your leisure if you want to keep your license. It does not happen. I can get into details if you want, but I think it's besides the point.
1) ARM sells "ecosystem". They can license their IP because licensees know that, once they buy into it, they benefit from a unified ecosystem. Developing and, more than anything else maintaining an ecosystem, is extremely expensive, messy, and error prone. ARM adds value by keeping that ecosystem in order.
2) ARM instructions have a fixed length of 4 bytes. This means that there's a limited number of instructions you can add. Intel, for instance, is different: they have a very complex encoding scheme that gives them, potentially, limitless encoding space.
3) If any licensee, specially a popular one like Apple, decides to take part of that encoding space for their own instructions... do you see the implications? Now, that space is unavailable to ARM. ARM, the owner and custodian of the ISA, are put in an impossible position: accept those instructions into the ISA (if Apple allows it, because now it's their IP) or lose unity in their ecosystem (fragmentation). In any case, they've lost extremely valuable encoding space and, more importantly, control over their own ISA.
And that's why not only ARM don't allow licensees to implement their own instructions, they will never allow it (generally speaking; they have reserved some encoding space in the M profile for custom instructions, but that's controlled customization in a profile that has fewer instructions to begin with).
