Presumably because none of what you said is necessarily true. Article 50 notification is not definitively "irreversible" -- since, for example, the article specifies that the notification to withdraw is to be "in accordance with its own constitutional requirements", or because the withdrawal proper is in two years from notification "unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period", both of which would imply firstly that:
(a) yes, parliament can be involved in the process because that's what the UK constitution would require at various stages, and
(b) yes, the notification isn't final, because the extension could for example by made indefinite if all agreed.
I am not, of course, a definitive legal authority, and it should be stressed too that I regard the idea of an "indefinite extension" (ie not leaving) as more of a technicality than something to aim for. Still, it does seem to me that regarding Parliaments, both the UK-wide and the Scottish/Welsh/Northern Irish parliaments as now irrelevant, is simply mistaken.
I also find it rather troubling how in their desperation to leave the EU, hardcore Brexiters seem to forget that the "Br" of Brexit means "British" and not just "English". The UK as a whole is leaving; the UK as a whole should be happy, by and large, with what that means and the shape that leaving takes, and if we push through the process with utter disregard for the other member countries of the UK then we may not even have time to enjoy Brexit before it's "Break-up" and yet another constitutional mess.