Why are on-hits and spell effects so unbelievably confusing and unclear?

I'm just going to list a bunch of examples of huge inconsistencies with these things. They are by far the least clear thing in the game and seriously need to be fixed.

• The new item procs on hitting a spell, but it's not a spell effect because it procs other spell effects.
• Yasuo Q applies on-hit effects but only on the first target hit. For example, you can hit a Fiora through her W if you have a minion in between the two of you.
• Clones that deal a smaller than normal % of AD still do full damage from on-hit effects
• Most physical on-hit effects proc lifesteal. There are random exceptions like Sheen that don't for seemingly no reason. Magic on-hits don't, but again, there are random exceptions such as Nidalee's Takedown that are exceptions to this rule for no reason. Lifesteal makes absolutely no sense and I guarantee you that literally nobody in the game can give you a perfect list of what applies lifesteal and what doesn't.
• Some abilities that you are told are on-hit effects are actually single target spells that proc spell effects. There is absolutely no way to know which ones unless you go and look up this information from outside sources.
• All damage from pets that aren't basic attacks proc spell effects. There's more rules to this but they're so confusing that I'm looking them up now and still don't fully understand.
• Autoattack modifiers can apply either spell effects or on-hit effects with no clear way of knowing which. Lulu's passive applies neither
• Every ability that is triggered on-hit applies spell effects except for Shyvana's.
• Some items are actually on-attack effects instead of on-hit effects. For example, Wit's end will still steal MR even if the attack is avoided in some way.
• Fizz's Q applies both on-hit and spell effects. The physical damage applies on-hits and the magic damage applies spell effects.
• Kayle's Righteous Fury is absurdly confusing.
• Some abilities convert all damage dealt into magic damage, even from on-hit effects like Sheen.

This isn't an all-inclusive list most likely, but should give a pretty good idea of how unclear and confusing these things are.