I won’t quote everything. But what always strikes me: when you post a fact you’ve learned here, you are treated somewhat reproachfully in tone – even though it’s not your own opinion, just the information being shared. Yes, it’s clear that the builder is not pleased when boundaries are set. It’s just an explanation – how the builder or the OP here then deals or could deal with the information is their business. You should just take in the background for yourself, but not immediately condemn the writer.
Where is the problem?
The problem is simply with the insurance. If a builder could enter all houses, then they could also steal something there. There would be no control. It could also affect your house. Why should neighbor y be able to enter your already locked house at your place, neighbor a? Maybe you already have your own tiles or fittings or tools stored there?!
That would be completely irrelevant to me as a builder. What if I want to perform work myself?
That is only possible by arrangement with the general contractor. That’s how the standard GC terms and conditions usually are. Performing own work is not forbidden at first. But the GC should be informed if you are intervening in the contractor’s work. By arrangement, there will then also be a key. After all, unsolicited own work can also be seen as tacit acceptance.