Not to defend the Indians too much, but I don't think that's true. Northern India and Pakistan were conquered a fair number of times, but I think the whole of India has only been conquered once, by the Brits. Maybe twice if you count Delhi Caliphate in 1400 AD or something like that, but by then I think that was a kingdom of native Islamic converts, not foreign occupiers. Indian kingdoms have spread out from India and had territory carved off too, but that's obviously different. Most invaders evidently make it maybe a third of the way in before they decide the rest of the subcontinent is really not worth the trouble.
Not to defend the Indians too much, but I don't think that's true. Northern India and Pakistan were conquered a fair number of times, but I think the whole of India has only been conquered once, by the Brits. Maybe twice if you count Delhi Caliphate in 1400 AD or something like that, but by then I think that was a kingdom of native Islamic converts, not foreign occupiers. Indian kingdoms have spread out from India and had territory carved off too, but that's obviously different. Most invaders evidently make it maybe a third of the way in before they decide the rest of the subcontinent is really not worth the trouble.