Preseason is always shit for balance. Also over a year???? Remember "ADC in 2017" in the early portion of this season??? When ADCs were literally shit besides the ones who had pre 6 utility.
ADCs and Supports are 40% of a team condensed to 1 lane. So yeah if that lane loses it hurts way more than say Top lane. However, ADCs are the primary ranged physical damage dealers and supports are most of the time the bulk of the utility. Structures are taken most often with physical damage, and utility is what decides even fights. So yeah, if your main damage towards structures and the bulk of your teams utility are weaker than the other teams, you tend to struggle winning the game until they catch up. This is normal. ADC is arguably one of, if not the most, item/gold dependent role. So if behind it really matters. The support is likely the least gold/item dependent role. So the lane balances out gold wise, but all the gold is mostly on just 1 of the 2. So if the ADC cant get gold, they are actually worse off than other lanes.
Say they need 1.5x the normal gold of other lanes to work. If behind, they may not even get half of the gold they need because of kill threat. And being that they offer almost entirely damage from items with their poor base stats, not having them is a massive issue for them. Because they take the gold for supports, when you are denied farm, they miss more than the other lanes.
Basically, many ADCs need a lot of expensive items or at least 2 items to do good. So if one team gets ahead, they get items faster and thus put the other behind by denying them farm and/or killing them (aka snowballing). Currently the preseason worsens this.