I trust my partner completely. We never mess with each other’s phones. Ever.
Let’s say you have two people. They both trust each other 100%. They are both also completely worthy of that trust. They never betray each other and never willingly do anything to hurt each other. They have absolutely no jealousy, curiosity, envy, or any other negative feeling between them. Two peas in a pod. The best couple in human history.
Sounds good, right? Both people can share all their passwords, phones, whatever.
The attack surface has just been doubled along with the target value. A third party malefactor can now choose just one of two people to compromise to gain access to both. How awful would it be for someone to have their stuff compromised even though they did everything right? Their partner had one slip up forgetting to lock the phone. The partner got socially engineered. The partner installed a malware. Now they’re both screwed. They could have protected each other from the rest of the world by keeping their things separate.
Circumstances change. People change. People have deep dark secrets. Trust levels change. People get married and trust each other completely and then surprise, they’re stuck in abusive relationships. This is sadly extremely common. The people it happened to once trusted their partners completely. I’m not saying to be paranoid, that’s just going to be a self-fulfilling prophecy. I’m just saying that these security practices are low effort common sense, like locking your house. Even when risk is low, it’s not zero.
There are also many scenarios that are unlikely, but not unrealistic. Partner was actually an international spy? Partner was a thief playing the long game to get access to bank accounts? Partner gets interrogated by law enforcement and coughs up the passwords? It happens more than never. Very unlikely, but protecting against it is so easy to do. No reason not to.
Strict digital separation also serves as a canary in the coal mine. Imagine instead of phones two partners each had one safe each. They trust each other completely, so neither should ever have any reason to suspect the other is keeping anything untoward in their safe. They would never even think to ask for the key or combination to the other person’s safe. Even just inquiring about the contents would be a signal that they are having feelings of suspicion, or worse. There’s absolutely no legitimate reason either would ever need to access the other person’s safe. As long as neither partner has ever even begun to attempt to access the other’s safe, that’s living proof of their trust for each other much greater than if both of them shared a single safe. If anyone does go near that line, it’s a helpful red flag that the relationship may be turning sour.
Lastly, it’s just a matter of the reverse question. You ask “Why not let your partner use your phone?” I say “Why do they need to?” There is no legitimate reason for these things to be shared. Everything that needs to be accomplished can be done without sharing. The cost of sharing is a reduction in security. What is the benefit? None. None benefit. Why reduce security for no benefit? Foolishness that will one day result in drama.