There are many reasons for this, but some major ones include modern Bible translations and not rightly dividing the Bible.

Modern bible translations: This isn’t the main reason, but this is a vital issue to be mindful of. There are over 200 bible translations, many of which have errors, such as the (NIV, ESV, NLT, NKJV, etc.) The best Bible translation we have to go off is the KJV; this doesn’t mean that the KJV and other modern translations never line up, but there definitely are errors within modern translations.

Side note: This doesn't mean that someone is automatically right about salvation solely because they use the KJV, nor does it mean that someone is wrong about salvation solely because they don't. However, the Bible translation someone uses can certainly influence whether they are correct or not about salvation. But overall, especially when comparing the KJV to other Bible translations, it is clear that the KJV is the perfect word of God.

Resources: The Bible translation issue can be very in-depth, but here is a link to a short video that further explains this topic.

Real Life Talks | Is the KJV the best version of the Bible?

The main reason why many people misunderstand salvation and believe in some form of a works-based salvation is that they don't rightly divide the Bible and don't look at it from a dispensational point of view, whether they realize it or not. Dispensations simply refer to how God deals with people differently in different time periods of the Bible. How does salvation fit into this? God's plan of salvation is not always the same throughout the whole Bible; there is such a thing as dispensational salvation. For example, we are under Paul's ministry for salvation today, which is salvation by faith alone with no works involved. But in other dispensations of the Bible, like in the Old Testament, it was a works-based setup.

The most essential thing for you to know is that the gospel of salvation for today is 1 Corinthians 15:1-4; that's the gospel that God revealed to Paul for us today (Galatians 1:11-12). It's essential for the Bible to be read in context; all of it is written for us to read, but not all of it is written directly to us. The Bible mentions dispensations four times and says to rightly divide in the book of Timothy.

The dispensation we are in today is known as the "Church Age." As mentioned previously, we are under Paul's ministry today for salvation. Paul wrote a large amount of the New Testament, including books from Romans through Philemon, which is the heart of New Testament doctrine for us today. You can go deeper than this, but to make it simple, the four main Bible time periods are the Old Testament, Church Age, Tribulation, and Millennium. Simply put, dispensational salvation can be separated into four categories: Old Testament salvation, Church Age salvation, Tribulation salvation, and Millennial salvation.

Verses about dispensations and rightly dividing: 1 Corinthians 9:17, Ephesians 1:10, Ephesians 3:2, Colossians 1:25-26, and 2 Timothy 2:15.

Side note: There are people with differing dispensational views who still preach salvation correctly for today. However, unfortunately, there are many who don't. While these are important topics, regardless of someone's dispensational or Bible translation viewpoint, the most essential thing is trusting in the finished work of Christ alone for salvation! The gospel of salvation is the most essential topic anyone can have correct. For salvation today, please understand that if anyone adds to salvation by preaching any form of a works-based salvation, they are wrong and are also rejecting Jesus' sacrifice. 

Resources: Here are two helpful in-depth sermons to better understand this topic!

Paul's Message

Following Jesus or How you are Supposed to Follow Jesus According to the Bible