This is just one data point, but I served on a NY grand jury a few years back. 30 cases, 27/28 found for indictment. of the 2 or 3 we dismissed, the accused testified on their own behalf during the grand jury process. In only about 5 of the 30 cases, the accused testified on their own behalf.

I don't know why most accused don't testify during the grand jury process, but without their testimony the process is obviously a very one-sided affair. As a result, there are many indictments and many plea bargains.