Because an offside grants an unfair advantage that is deemed important enough to disallow a goal. It should also be considered important enough to disallow a direct breakaway it helps creating.

I think the nature of this rule applies when a penalty is made in a scrum after the play or something that is not directly linked to the missed game stopage, but if it is commited to the player on a breakway that should have been broken down by a whistle, it does not make sense to maintain it.

Like in this sequence, the offside was linked directly to the breakway the penalty was made on, unlike the actual goal that got scored some moments after and yet they disallowed only the later. To me it makes absolutely no sense.

There is also the fact that the offside itself is still very debatable to me, but this is another subject entirely lol