Between the era of fixed capitals beginning, and the mid/late 19th century where the whole front could be manned, why didn't the main army of one country just bypass all forts and go straight to the foe's capital? Though it wouldn't be a silver bullet, I would have thought taking the capital would demoralise the enemy government, sow chaos in their administration, and possibly allow the capture of many key commanders. Did forts have sufficient ability to scorch-earth armies that they couldn't maintain such a march? Were capitals easily moved and hence taking the normal one would be no more significant than any other city?
For instance, I was reading Four Princes and it talks about Suleiman's advance towards Vienna, and he spent 25 days besieging Guns and it's 800 man garrison while he had 100,000 men. Why bother? Couldn't he just leave ~4000 men there to blockade them while his massive army heads for the real target?
Logistics for one. Those forts likely defend key roads into the capital or similar - bypassing them means your carts carry food, supplies etc. are going to at minimum take longer to get to the front. Wear and tear from going off road on your supply train might be a problem too.
If you end up in a siege at the capital, you might need to maintain supply for a very long time. You could leave a screening force to besiege the forts in the rear, have troops patrol the roads to stop people attacking your supply line etc... but now your spreading army out more and more as you advance. Troops left behind are also vulnerable to attack since they're away from the main force and its support.
If things go wrong at the capital, you'll have to fall back. And the roads might not be an option if you didn't clear the rear areas properly first. If you had captured key points on the way in, you could just fall back there and try and hold on.
Depending on the time period you mean, capturing the capital might not actially give you control on its own. Countries with feudal or other regional government arrangements may continue on in the absence of a centre, leaving your conquering army in the middle of a country, bits of which will stay in the fight around it.