Check you’re altering the relevant number here. If you’ve manually altered any of the staff spacing yourself, select those systems and (in Engraving > Staff Spacing) Reset Selected Systems/Frames. Also pay careful attention to the “Vertical justification” sub-section within Layout Options > Vertical Spacing. If these percentages are too high, relative to how much music you’ve got on a page, then changing the numbers at the top of that dialog (Ideal Gaps) isn’t going to do anything.
This has been done to death. The rationale for using spaces rather than inches/mm/pts in Layout Options is that spaces are relative to staff size, and inches/mm/pts are absolute. If you’d had the option of using inches in Layout Options, and then decided to change your staff size, all your spacing options would have to be changed.
That said, the team are clearly working on it. https://www.steinberg.net/forums/viewtopic.php?t=100053#p661439 is the first thread that appeared when I searched this forum for “spaces inches”.