Macbook lists all WLAN-networks of another machine. How and why was they shared? Maintaining my wife's MacbookPro, that never left the house over many years, i've found in the list of preferred WiFi-networks ALL networks, that i was ever connected to worldwide with another Macbook, even a complete new one. Also all networks, we was ever connected to with our iPhones are listed on this Mac.

We don't use a shared iCloud-account. The only thing in common is, that all those devices are connected with the same WiFi at home quiet often.

Although it's not an issue here for me ;), maybe it will be for others getting leaked their locations where they've stored a WLAN-connection that shouldn't be visible to colleagues, partners, children and so on.

My question is: how and why are the networks shared across machines and across accounts/different persons? Isn't it a privacy issue? I don't know if passwords to the networks are also present on her machine, because i don't know how to test it without travelling. If so, she could connect into the networks of several companies i have worked for recently
All stored wifi are stored in the user and system keychains. You can open up Keychain Access and search for all known / saved networks in the system and your user account. 

Repeat for other user accounts and you'll know how or when they arrived. 

iCloud Keychain syncing seems the most logical source along with not changing the default "remember networks I have joined" but rather than speculate, you can go to the source keychains.