Perhaps it's useful to provide some clarification here. As the other user stated, Linux is set up for multi-user setups and provides logical protection, but you seem to misunderstand how operating systems and file permissions work.

If someone steals your unencrypted hard drive and boots into their own operating system, they are able to circumvent all access control and permissions on your hard drive. This is because when they mount your hard drive your operating system isn't running; they're simply reading the stored data, so the access control and permissions set up by your operating system don't mean anything. This happens with ALL operating systems (Linux, BSD, Windows, MacOS, etc.). Logical protection like access control is only useful while the OS is running, and it cannot help otherwise.

This is why encryption is important, because it prevents unauthorized access when the OS isn't running. If you'd like to see just how easy it is to access unencrypted data, make a live USB and boot into it on any unencrypted computer (assuming you have permission to do so if you don't own the computer). You don't even need to extract the hard drive in most cases to read file contents, you can simply boot into a live USB. The only situation where this isn't the case is when USB booting is disabled in the BIOS and the BIOS is password protected, but you could always just remove the CMOS battery to clear the settings to bypass the BIOS password anyway.

Unencrypted data will always be trivial to retrieve when the attacker is allowed physical access to your computer.