I was crunching some numbers, let's see if my assumptions are correct (comment below if you disagree):
Assume you are the Hindu Shah of Afghanistan based in Kabul, I think medieval cavalry and gunpowder made infantries less effective in Steppe & mountainous https://twitter.com/Hastivarman/status/1301951012886835200

warfare (compared to interior India) especially when on the attack on an open field because of logistical reasons. Remember that the population was sparce, so I doubt you'd be able to field a bigger army than 20-30,000 troops, mostly light and heavy cavalry and artillery.

Also, you can't commit your entire army at the front, you have to keep some in reserve to guard your main city/fortress.

So why didn't they seek help from Hs in interior India?

Let's look at the distances. Distance from Kabul to Lahore, assuming mountains and valleys, is 600 kms minimum, it would be larger in the winter. Again, mountains make it logistically difficult to supply and reinforce the troops.

Afghanistan is not a very fertile piece of land either, so you can't support reinforcements locally beyond a certain extent. For all practical purposes, you are on your own.

Even with a hypothetical Grande Armeé of bvll chad Hindus, it'd be difficult to attack Ms of Iran and

Central Asia because of the same logistical issues. You'll have to field a very large cavalry, probably more than 50,000, along with artillery, and supply them with food etc. because chances of foraging are low (soil isn't very fertile) and very very hostile population...

as you have incurred the wrath of the Ummah. Remember that your troops, since now you have gone on the offensive, are now more than 1,000 kms away from Lahore, which is the nearest city to have a reasonably large population and high agricultural output.

Supplies would have to come from Lahore via land which is mountainous, can't do it effectively in the winter, so there is a time limit to the campaign which has already seen large distances crossed in an increasingly hostile population which isn't even properly settled.

Steppe tribes can deploy scorched earth tactics and absolutely ruin your supply lines, after which, you are sitting ducks to an army raised by the Ummah to fight the kuffar Hs.

Hence, defending your fortress would be your logical goal and betting on offence beyond a certain

distance as a strategy to defend yourself would be a terrible idea.

Simply put, every war has a cost to benefit ratio which the Raja had to estimate before he set out campaigning. It just didn't make much sense for Hs to invade C Asia from India from that standpoint.

Did we underestimate P threat?

Probably true.

But was victory guaranteed had we taken the war at their doorsteps?

Absolutely not, and it would just trigger their Ummah more in my opinion.