This idea doesn’t seem obvious to me. The biggest states are:

  California
  Texas
  Florida
  New York
  Pennsylvania 
  Ohio
  Illinois 
  Georgia
  North Carolina
  Michigan

Which major industry unites them? Which major industries are not represented? You have tech, finance, automakers, agriculture, tourism, and oil. Maybe not much mining and fishing? Probably something else, but these are very different states.

It's worth adding that those states aren't even close to as homogeneous as most tend to assume. Only 60% of California voted Democrat in 2016, and you have to assume the turnout for Republicans would be higher if it wasn't guaranteed that their vote (for president) was meaningless. With the current system, you can cater only to the populous parts of California and win the whole state, and that goes for most other states as well. With a popular vote, that ~30 to 40% of California is relevant again, along with significant chunks of other states.

It's also worth noting the math really doesn't work out, the electoral college makes it easier to win the whole thing while carrying less states/votes (Which really shouldn't be surprising). With both the popular vote and the electoral college, you only have to take the 10 most populous states to win the whole thing. But the difference is that, with the current electoral college you generally only need to get 51% of the vote to win the whole state, which is significantly easier and turns states like California from a 60% to a 100%. With the popular vote, to actually win in that fashion you would have to take almost every vote in those states, which is practically impossible considering the diversity.

If it takes about visiting why to the campaigns spend so damn much time in Iowa?

Iowa and NH are targeted because of early caucus and primary, key in nomination contests because perceived momentum drives donations, media coverage, and later votes.

An early primary, I believe.

Yes, but if it's really about ad spend and not being physically present, why do they spend so much time being physically present in a tiny state?

Because of primary campaign dynamics where those are literally the only stayed voting at the time, and underperforming there means you have no money, no positive news coverage and no reason to campaign anywhere else. So all energy goes into them at a certain point in the nomination campaign. (As a rough approximation.)

Totally different dynamic than general elections, which happen all at once rather than different dates in different states.