I am a comcast customer in Baltimore County. I do not get the Washington D.C. Network Affiliates? Why? A friend of mine in Calvert County is a comcast customer, and he gets both. Another friend of mine is in Howard County, and he gets both. Why? Can I pay for both?
Baltimore County, MD is part of the Baltimore television market. The Washington DC channels are not required to be carried on cable there.
Television stations almost always elect "retransmission consent" when they are carried on cable, since it results in the cable companies paying them each month (per subscriber) to be carried on cable. They can choose where to be carried and where not to be carried. Sometimes, the "in-market" stations can block an out-of-market station from being carried on cable to protect their own viewership.
You should be fortunate you get local channels from your own state. There are many "orphan counties" in the USA that are stuck watching affiliates from a different state, and have to watch political news/ads that isn't relevant to them.
In the case of these two markets that are very close together, the immediate areas can have exclusive rights and help black out the affiliates in other markets. This is not the case in some of the outer areas like Calvert County (who primarily gets DC, but has some Baltimore stations) or for Harford, Howard and Anne Arundel counties (who primarily get Baltimore stations, but have some degree of DC stations).
Depending on where you are in Baltimore County, you may be able to receive DC network signals from an antenna. Your mileage may vary depending on your elevation, proximity to the towers, and you may get better reception from some DC networks than others, based on their transmitter location.