I think you need to understand just how small of an improvement you're talking about. Suppose you have an ATK champ and you upgrade a glyph from 3% ATK to 5% ATK. Assume the champ has base 1500 ATK. That translates into a whopping 30 extra ATK. Say you have 250 CDMG and 100 CRATE, and you're using a move that has a 4X ATK scaling. Also say you had 5k ATK before, and after upgrading you have 5030 ATK. Finally, say you had an ATK up buff. Before you'd do 5000 * 3.5 * 4 * 1.6 = 112000 damage, before other modifiers. After, you'd do 5030 * 3.5 * 4 * 1.6 = 112672.
Are you going to notice 672 more damage in that hit? Remembering that number will be even lower due to DEF? Now apply that same logic to 9 items - say all 9 of them gain 30 ATK. That's 270 more ATK, resulting in 5270 * 3.5 * 4 * 1.6 = 118048 damage. Now we see a bigger increase - about 6048 damage. Again though, that number will be lower due to armor. But even just taking it at face value - on a CB team, over 50 turns, that 6048 damage will mean a total of 300k more damage. Will you notice if you go from 70M to 70.3M?