There is no way to conclusively prove that the ball went out of play. Goal line technology always uses that angle from directly above the ball due to the spherical shape of the ball.
Many decisions look over the line but when you take into account the wider part of the ball that's not touching the pitch then often the ball is still in, even when it looks out because, obviously the bottom part of the ball touching the grass can be over the line.
Not saying that is the case here, but the call is so close it could easily be the case and they fact that the goal was ruled out based solely off an "eye test" from the best available angle is ridiculous.
From a biased point of view, when these decisions go against you it feels horrible and you question why you even bother putting all that time and energy into supporting your club. VAR was supposedly supposed to remove that frustration, not compound it.