Why would a modern home have baseboard heating?
Baseboard heating vs. forced heating and other options is not dependent on class of home.

For example, some people prefer the radiant heat, as in general it actually is far more comfortable than forced air (many opinions argue in both directions).

Some people have allergies, and thus, actually prefer radiant heat.

To your point, forced air affords central A/C options. I own a NE home I am selling right now, and guess what? for 9 years we wished we had forced air. Why? Solely for A/C for .... two months of the year, at best. We did not mind baseboard at all.

When it comes to a builder, they are all going to have their reasons. Some might place the above considerations high on their list, but more than likely it is based on price and availability.

I'm not solely discussing price of the equipment - but also of installation. Not to mention the inherent design/layout considerations that have to be thought of to run ductwork vs pipe.

on the topic of availability, I found that it seemed MANY people in NE still rely heavily on Fuel Oil. I intentionally converted my home to High Efficiency Natural Gas but it was nice to have radiant heat nonetheless. Especially in the north east. So contractors in general in NE I suspect (strongly) use radiant systems because they are plentiful, still very popular, and are not cost prohibitive to install.