The first point here is that not all failed subs are equal. Big difference between for example Vitor's armbar attempt on Jones and a half-assed guillotine someone tries as they're getting taken down.
Sometimes failed joint submissions or chokes can still do damage, but that's hard to score.
Subs can also take a lot of energy to defend, so they're offensively significant as a way of exhausting the opponent. This is also problematic because the reverse is also common : you can burn out your arms trying to crank a sub and that's tactically an advantage for the defender. A better grappler can and will bait their opponent into doing this, eg Khabibi and Dustin's guillotine.
Sub attempts from a dominant position that don't compromise your position (choke attempts from the back, Kimura attempts from side control, arm triangles etc) can show dominance and control of the fight. The guy who's doing them is pretty clearly winning and showing that given time they could realistically finish the fight, especially if they maintain the position for a long time. It's hard to defend indefinitely.
In general this is a pretty nuanced question and there's no universal answer.
