I could go on and on about the importance of contacting a lender prior to looking for homes, but the summary above is really all you need to know. People are busy, time is valuable, etc...
Real estate agents are paid commission. Spending weeks, or even months, with a client who isn't qualified to buy isn't on the top of their list of fun ways to spend a weekend.
More importantly though, looking for a home without first being pre approved is a giant waste of YOUR time. Maybe you have great credit and a good job and assume you will have no trouble finding a loan. You probably do qualify. But what is worse than finding your dream home and discovering while yes, you qualify; the amount for which you qualify is not enough?
Preventing this is easy: Get matched with lenders here.
Once pre approved you will be provided a pre approval letter to present to sellers and real estate agents. They will know you are serious about buying and have taken the necessary steps to ensure your offer will close.
Next: VA Funding Fee Chart