1. Marketing: GameFreak wants at least a group of easily recognizable 3-stage pokémon that EVERYBODY will be familiar with in order to make marketing easier, so they have to be available early and you don't have to scour the region to find them. The starters take the longest to design for this specific reason, they are what GameFreak hopes gets the most revenue from the games: starters and box-legendaries. That's what sells the games along with new regions and then you get into stuff like story and the more obscure dex-entries.
2. Learning-curve: Newcomers can easily understand that water douses fire, fire burns grass, etc.
3. Difficulty-curve: You can't get your favorite (or choose any) at the beginning because that will make the game off-balanced. My favorite could be Rayquaza, but the moment I get to pick a Rayquaza at the beginning then the entire games difficulty curve becomes moot. Even if your favorite is Lucario and you get a Riolu, then the game will get far too easy far too fast. This is why the hand-outs in XY were such bad moves on GameFreak behalf. The Kanto-starter handout, followed almost immediately by a free Mega Lucario. The rest of the game was incredibly easy afterwards (alongside with the xp-share being completely whacked). The game is just too easy when you pick Froakie, get a Charmander two towns down the road and then get a free Lucario yet again two towns further down the road.
Re 1 - Yes that makes sense from GameFreak POV, except they don't need it. They could still slap an image of previous startes and everyone would go "Oh a new Pokemon game." Fans will buy the new game anyway and newcomers are actually more likely to recognize classic Pokemon.
Re2 - Um, that is not even learning curve, that is just one NPC telling you this stuff. Since the game insist on unskippable tutorials, might as well make that explanation part of it. There, resolved.
Re 3 - OMG you took me literally.
Obviously you wouldn't be able to get legendaries, or mega stones. What you imagined a National pokédex list and a player clicking on it or something? Just offer the player selection of stage 1 of Pokemon who are available in the game.
No it is not going to be any easier. In fact starters are usually the ones who are easy to train, get powerful moves and early evolutions. And no offense to anyone but with
free unlimited grinding and static enemies and bosses, one should be able to win with six Raticates. You don't even have to be competitive, just level them high enough.
This is also very simply resolved by "recommending" the most balanced Pokemon to the player first (new starters), and letting them select anything else if they insist oherwise. This way people who like the concept of starters can still have them and the ones who want either challenge or something else have other alternatives.
This franchise most amazing feature is that it has 800+ virtual creatures that you can play with. But instead of using that to their advantage and showing them off, they actually limit the number of them. 800 Pokémon, and I can choose from whopping
three of them at the beginning of th game :/
It might add variety, but the function of the starters is to explain how the game works in one short battle and fire burning grass and water dousing fire is just so much simpler to understand
And why do you need starters for this? Because that could be resolved with a single tutorial. Also as of gen7 the move menu TELLS you what is and isn't super effective. Why would you tie a very simplistic learning process to a choice with a Pokemon that you are stuck with for a remainder of the playthrough? You can have those separated.
Anyway, those are just my two cents. It is not like the world will end because we have yet another grass/fire/water starter to choose from, it is just somewhat disappointing, because it is getting stale.