Because after certain levels in D&D, spell effects become literally time and world altering.
I mean, effects from 7th level spells:
You conjure an extradimensional dwelling in range that lasts for the duration. You choose where its one entrance is located. The entrance shimmers faintly and is 5 feet wide and 10 feet tall. You and any creature you designate when you cast the spell can enter the extradimensional dwelling as long as the portal remains open. You can open or close the portal if you are within 30 feet of it. While closed, the portal is invisible.
This spell reverses gravity in a 50-foot-radius, 100-foot high cylinder centered on a point within range. All creatures and objects that aren�t somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell. A creature can make a dexterity saving throw to grab onto a fixed object it can reach, thus avoiding the fall.
This spell instantly transports you and up to eight willing creatures of your choice that you can see within range, or a single object that you can see within range, to a destination you select. If you target an object, it must be able to fit entirely inside a 10 foot cube, and it can�t be held or carried by an unwilling creature. The destination you choose must be known to you, and it must be on the same plane of existence as you. Your familiarily with the destination determines whether you arrive there successfully. The DM rolls d100 and consults the table.
All superfun if you play at a table and a DM arbitrates this stuff. In a videogame, this would be, indeed, really hard to pull off and still maintain any kind of balance. Summoning a house or teleporting to the Druid Grove whilst on top of the Elder Brain would suddenly make for a whole bunch of questions to ask before implementing. Well, you, would say, just alter it or leave those spells out - but that would diminish the addition of those levels entirely. Sure, I agree that character building is really fun, but that doesn't come from levels alone. Never mind the amount of complaining you get here if you change something from the Core Rules.
A party at level 12 is an entirely different animal at the end of Act 3, than a level 12 party at the start of it, just by sheer item combinations and spells I've found, Illithid powers etc. Progression and things to do don't relyi solely on character levels. Lastly, building within constraints is still building.
