Patent law, like most other areas of law, has been shifting over the years. One of the more recent changes made the United States a "first to file" country. The grace period is nearly non-existent, and inventors and business owners need to file first to make sure their designs are secure.
They don't always do this, sometimes just due to ignorance of the law. They may feel that they have more time, only to find out later that they've lost out due to their procrastination. If someone else beats you to the patent, it can crush your plans.
Other times, inventors don't file because they're simply too busy. For those just starting out, 24 hours in a day often doesn't feel like enough to get all of the work done. Filing a patent gets put on the back burner in favor of other tasks. This is understandable, but it's also incredibly dangerous.
In still other cases, the money just isn't there. Inventors decide to wait for everything to come together. They have plans, but they need to seek out investors, apply for loans, and do much else to get the financial side of things in place. They decide to wait to file until this is done. Again, there's a high level of risk here.
Remember, being the first to file can be the difference between seeing your plans come together and watching someone else profit. Always be sure that you know the current U.S. patent laws and how any changes are going to impact the way you approach your legal filings.
Source: IPWatchdog, "5 things inventors and startups need to know about patents," Gene Quinn, accessed Sep. 09, 2016