Keep in mind that you're doing this because you're mating aluminum and steel. If it's a tight fit, chances are the bolts will seize a bit simply due to the fact that, as you rub the aluminum, you strip off its corrosion layer.
The other part is the steel/aluminum parts in contact react. This gives a quick, simple explanation.
https://www.galvanizeit.org/design-a...als-in-contact
One thing that can help is a little bit of non-metallic anti-seize compound. Dab a little on the thread of the bolts.