If you're looking to use that 100k soon, maybe for a new house, or saving to start a business, then you want very safe and liquid options. 4% is close enough to other safe options that if it's easy then that's good enough for some people.
If someone is sitting on 100k in savings, there isn't much of a practical difference to moving it to a money market. The rates aren't significantly different. Some banks have higher savings rates, some higher money market rates. That makes other things your deciding factors such as the trust in a brand, quality of life, ease of use.
Remember, taxes are designed to benefit rich passive investors, so you gotta act like one.