Reading text alone, it can ba hard to differentiate between seine wanting it for a personal preference, and for an advantage.
If it's an option that's clearly available for everyone, then there is no advantage to be had.
NOT including an option actually creates more room for people to get what could be considered an unfair advantage.
If FOV is locked and defined by the vertical axis, then someone could gain an advantage by using a wider monitor or multiple monitors. If FOV is locked and defined by the horizontal axis, then someone could gain an advantage by using a narrower aspect ratio - there are some games where I play in a pillarboxed 4:3 resolution in order to make the FOV more comfortable.
Lack of FOV option also leaves open the route for more determined players to gain an advantage by changing it with console commands, config files, or outside programs like Widescreen Fixer. Make it an in-game option, and any concerns about cheating with "fov hacks" would disappear.
Without FOV options, you also create accessibility problems for people with non-standard aspect ratios. If a game is designed for 16:9 and the FOV is locked on the vertical, then someone stuck playing on a 4:3 screen will effectively get a much lower horizontal FOV than they should. If the FOV is locked on the horizontal, then your game is essentially unplayable on ultra-wide or multi-monitor setups because the vertical FOV will shrink so much.
The idea of locking FOV to create a uniform experience for everyone is BS, because it's only one factor in an equation that includes screen size, screen distance, and aspect ratio.