Easier for people not used to dice systems to understand. It’s immediate and obvious, most non dnd players can grasp it at a glance.
I repeat my point: you could have the option of toggling it off. Both types of people would be happy, then - those who don’t like percentages and those who do.
And I repeat mine... If someone is interested in playing D&D at the table, as BG3 will undoubtedly foster for many, showing the target d20 DC in the combat tooltip, which is essentially the same exact thing as what the dialogue UI is doing, will (most likely) ease the pain of understanding how attack rolls work for a tabletop game. Showing percentage to hit in combat and a target d20 DC in dialogue is inconsistent and more confusing for people looking to play at the table.
You seem confused about what the game actually tells and shows you.
You don't know the DC of a dialog check before you choose it. That's why you're only told what your flat bonus to the d20 roll will be and if you have advantage or disadvantage. In combat you know everything you need to calculate the percentage success rate: Targets AC / saving throws, your attack roll mod / spell save DC, whether you / the target have advantage / disadvantage.
If you want to know what your attack roll or spell save DC is, just look at your character sheet / inventory or whatever. The green/red chevrons tell you if you or the target have advantage/disadvantage. Everything you want is already available?