That reduces entry points but you will never ever perfectly seal every nook and cranny. All it takes is one insect to get in to be able to replicate if tmthe stuff that draws insects in is present.

People regularly live in homes without.insect infestations despite leaving windows wide open without screens. They keep the house tidy so they aren’t feeding anything that gets in, they close the windows when it is dark out to avoid light drawing things in, and they make sure to get rid of waste. If there is nothing for scavaging bugs to eat there won’t be anything for the predators to eat and so on.