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Tldr; Quality not Quantity.
Because for 8km race pace should feel relatively comfortable still and not leave you knackered (it's not 5km pace it's 21km pace), it is when you scale it up it'll begin to feel uncomfortable and knacker you out - which it will do on race day to an extent, but not be as uncomfortable if arriving at the start line either un-trained or over-trained.
Making it uncomfortable too often in training will result in the latter as fatigue builds.
Keep in mind those race pace sessions are one piece of the bigger jigsaw.
Yeah this makes sense and is valid, but the half marathon plan referenced has long runs up to about 20km (at 'long run'/easy pace), but then running at race pace is only ever done up to a total of 8km in a session. As @yoshy said, I am struggling to see how running 8km at pace would be sufficient preparation to then go and run 21km at that same pace.