That's how the system was designed. Devolution was supposed to be a release valve for dissatisfaction with Westminister, and ensure some comfortable jobs for those on Scottish Labour's B team. Donald Dewar (who was a seriously clever guy) devised the system in such a way so that it was thought impossible for the ScotNats to get anywhere near power.

If the Scottish people get serious about supporting independence, then the status quo is going to make it very difficult for Scotland to leave. A vote in Holyrood isn't going to cut it. There is no magical legal jujitsu, no secret safe combination, that can be used to force the UK government to recognize the Scottish people's desire for independence. There will not just have to be a vote, there has to be a movement for independence with a force that the UK government has no choice but to accept.

The status quo making it nigh-impossible to change the status quo isn't a bug, it's a feature. The status quo seeks to defend itself at all costs.