It’s a lot fewer keystrokes for MS (Morgan Stanley), GS (Goldman Sachs) and MSFT (Microsoft) than it is for AAPL, but it’s a force of habit for some. Once you’re used to referring to firms by their ticker symbols, you do it all the time.
E.g. an ex trader friend still says “spot” instead of “point” when referring to decimal points, even if talking in other contexts like software versions.