Because...
a) 60A may not be adequate for a lot of systems
b) doing something 'cutesy' might be a lot cheaper. Not all houses have meter/main combos. (And by the way, line side taps are about following the rules, not getting around them, specifically the 120% rule.. Otherwise we'd just stick higher rating breakers in load centers.)
All the houses here have meter main combos. The title of this thread was getting around the 120% rule without line side tap. There is no way around it. You may have to derate the main breaker and depending then the size of your bus will still dictate the amount of solar that can be installed
John Wiles did not write the code section. I can see you didn't really read my previous post. I agree that the code is the code, and the code is not whatever John Wiles says.
I'm guessing that if he didn't write it that he had something to do with it, otherwise why would he be writing articles explaining it?
Give me a logical code reason to agree with you. Read the 690 definition of a system and tell me why one inverter isn't sufficient to call an installation a 'system'. If I install a string inverter on the main house and a micro-inverter system on the detached garage in what manner is it sensible to call all of that 'one system'? If I have a wind-turbine inverter and a solar inverter are they 'one system'? If yes, how are these the same 'source' (the word used by the code)? If not, what possible safety justification is there for treating interconnection differently than multiple solar (or wind) inverters?