There’s so much more to open source than what fits into a license.
Open source software (OSS) in spirit has existed since the 1950s, when researchers and academics shared source code with each other, treating code as a common good. After a court decision rendered code copyrightable in 1984, the Free/Libre/Open Source Software (FLOSS) movement started in earnest. Just a few years later, the first license that embodied the spirit of open source was created, though it is debated whether this was the GPL or MIT.
Open source is software where the code is available for anyone to use, integrate, examine, modify, and distribute.