“Right click” bias in Dota2

I'm quite causal Dota player, so I might not know much, but why is there such a big bias for "right click" heroes vs spellcasters? Personally I enjoy playing spellcaster heroes, such as Zeus, Rubick and Skywrath Mage way more than heroes that mainly just focus on right clicking. But it feels that you are in such a big disadvantage, because almost all the items are made for right clickers and you cannot scale into the late game.

Personally I've always thought that BKB is a ridiculously op item, at least nowadays it only reduces the damage you do to them by 50% rather than full immunity like before, but it still makes them fully immune to almost everything for 9-6 seconds with no downsides (other than the duration is lower next time). If you compare this to what you can build to protect yourself from these right clickers? Euls, which makes you immune to damage for 2.5 seconds, but its basically a self stun as you cannot do anything while you use it. Or ghost scepter/Eblade which gives you immunity for 4 seconds, but makes you take 40% more magic damage. Both way lower duration and big downsides when you use them and both are countered with Nullifier.

There are only few spellcaster items compared to what right clickers can build and most of them don't build up to anything bigger for late game. Aether lens, Veil of discord and phylactery, none build up to anything bigger and cannot really help you win in late game so they are basically support only items. Where as right clickers were given new combinations allowing you to upgrade your Diffusal blade and Echo sabre into the late game. Even Rod of atos which I would think of as a spellcaster item, was made to combine with maelstrom, making it yet another right click item. They even introduced Mage slayer into the game, I guess spellcasters were just too op.

Even Puck is usually build as right click hero nowadays because of the shard and Aghanims and Zeus was given an option to build more into right click with the new shard (I know it's more of a meme and doesn't really work, but it gives you an idea that they want to make all the spellcasters right clickers rather than giving them opportunity to scale as spellcasters.)

Is there a reason why we cannot have more spellcaster heroes and items?

Why doesn't intellect give you spell damage so you could scale into the late game?

Am I stuck only playing pos4-5 if I want to play a spellcaster and never scale myself? To be spam reported if I don't only focus on building items such as glimmer/force to save my precious carry so they can have more fun?