Why are boys so much harder to potty train

I grew up with two sisters, no brothers, raised to girls, have two granddaughters and now have three toddler sons. Two of my sons are 3 the other 2. I am having the hardest time potty training them. I remember my girls using the potty on their own by the time they were 2 yrs. Everyone keep telling me that boys are always harder to train than girls. I always took this statement lightly. I guess they were right.

Any suggestions:-)