I don't see what the problem is. If WotC were to include the entirety of the D&D game in the "starter" set, what would be the point of having the three books?
Oh I am sure other posters have already answered this and I haven't read the posts yet, but I have to say something even if it repeats others posts.
Starter, beginning, entry level. The starter should be just that.
Take for example Games Workshop products for Warhammer (FB/40k). They don't include paints or glues for various reasons related to ages and such so they will be a needed extra just like a surface to play on. So they can be discounted for this example as oddities that D&D doesn't need or have that has age restrictions on chemical products in some areas.
This "starter" for Warhammer includes the rules to play and even how to make a little army with the models given. They don't just give an army list but tell you how the army list is made from the rules. This is done because anyone playing this starter may wish to buy more of the full line of products. The rules included are for the pieces included, and not for every models allowed in the game form every army, only the select few included in the "starter".
With D&D, this could also be done by including rules for creating characters up to a certain level. Seems the favorite level at WotC is level 3 for "demo" products, so we will say level 3.
This would give away ALL the rules to creating characters at a level higher than level 1, but nothing higher than level 3 as the items above level 3 would not be included in this starter as it is just a starter, a primer for the game to introduce people to it.
So if it gave the basic creation rules in an actual D&D product in order to allow people to grasp how the game will be played after the starter is over, then they can go on to purchase the rest of the game, the core books.
Also you have replayability within the game because you can try making other characters with the limited rules supplied and see what happens and how you like it.
Since only some people play the game not caring about their own character, a lot more play with interest in their character, and this gives new players the ability to see that they control more than just the few premade characters. It is also a big part of the whole of D&D, and you must make your own characters more often than one will be made for you.
Not to mention for those of us in the know that is the first thing a player does, and where they "start" to play the game by making their own character.
So it doesn't have to give level 30 items, feats, paragon paths, etc to give enough to show the game basics.
So the point in having, someone buying them after the starter set, the core books is to be able to play more than just those levels/monsters/etc included in it.
What level worth is included or should be in a starter? Who knows. Depends on the adventure included and if it offers any level advancement at all, and if level advancement is included then why leave out the initial level creation?