It's often not a conscious thing, but more along the lines of "Oh, I have a Contact page on my website," or "All they have to do is hit reply to my email," or this classic from the Shoot Myself in the Foot School of Marketing: "They have my contact info."
It's critical that people be able to contact you, easily, in a way they (not you) are comfortable with.
If you make them jump through hoops just to reach you, even a small hoop, they may not stick around long enough to buy something (or answer your email, or help you with your problem).
If you make them even a little bit uncomfortable (or frustrated) because you only give them one way to contact you and it's something they don't like (they feel uncomfortable with website forms, for example), they will find someone else who they feel better about.
I've said it before and it bears repeating: Put complete contact info on everything. Otherwise, you're just sabotaging yourself.
Soooo....For emails, that means a signature block at the bottom of every email. Set up your email software to put it there for you automatically every time you write an email.
In your email signature include all the ways people can find you: your name (seriously), your company name, your email address, website url, phone number, address (if you have a retail location), and social media links.
You can include an image in your email signature (like your company logo) and your motto or tag line to help them remember who you are and what you do.
For websites that means not only a contact page (or contact info visible from every website page), but multiple ways for people to contact you so they can choose the one they feel most comfortable with: Email, phone, website form, social media.
Just do it. You'll be amazed at how much more responsive people are!
Want help? Contact me at 310-480-6738 or email me. (Notice how I gave you two options in addition to the other options you see here?) :-D