I agree that it's not unusual to not have close friends as an adult.
I had very close friendships during school, but once we became adults, we went our own separate ways and lost touch. I never formed closer friendships after that. In my case, it was because it feels like too much work.
After doing everything I have to do, I don't feel like making the effort to be with other people, because I don't enjoy it! As a very introverted person (sorry, but it's true-- and relevant here), I'm don't need much "people time" to be happy. I get all the social time I need from my husband and family (parents, sibling, etc.).
If that changes, I'll probably find that my priorities shift and I'll put energy into finding a friend. It won't have the same history as a 30- or 40-year friendship, but I believe you can make good friends at any stage of life.
I understand that if your husband is your best friend and he dies or leaves, you are suddenly missing a huge chunk of your support and there will be loneliness until you find another friend-- but losing your husband is always going to be life-altering and challenging. If I'm only maintaining a friendship as an insurance policy against being left alone in the event of tragedy, it's not going to be much of a friendship.