Everyone's talking about the merits of the ADC, not the bot lane mechanics. Regardless of comp, why are their no other candidates for the bot lane position?
Well part of that comes from the XP sharing that happens in the duo lane. Because an ADC relies more on AA rather than their abilities, they are more item dependent, rather than level dependent. Which is why they can deal with the XP deficit.
Which begs the question, Why not skirmishers? Skirmishers were historically referd to as "melee ADC's" so logically, they should have that same advantage. But so far, melee ADC's can't seem to handle the ranged harass that the marksman puts out early. Yet when we give melee more tools to counter range, (Like Yasuo's wind wall) They only become better if not frustrating in the solo lane.
To answer your question "why can't Riot Rework Bot lane?" It's Simply easier said than done. The Marksman's synergy with the duo lane pretty much existed organically. So far the only other Organic exception that popped out was the Ziggs bot lane. Outside of that, your going to have to promote items and champions, designed for the Duo lane, in such a matter that they don't become oppressive in the solo lane. So far, I can only find 2 exceptions to that. Mord, who was Riot's first experiment at an alternative bot laner, And the relic shield line, which can help a melee bot laner sustain, will generating even more gold for your support. But this is not nearly enough. Riot pretty much stated that as a result of their Mord experiment, Alternative bot laners were going to need more support with itemization.