That's not how this game works.
The Players' Handbook says:
As your character goes on adventures and overcomes challenges, he or she gains experience, represented by experience points. A character who reaches a specified experience point total advances in capability. This advancement is called gaining a level.
This has been a hallmark of D&D since the earliest days.
If you need an in-game explanation, here are a few.
The gods' power is not infinite. In the DMG, under "Gods of Your World", it discusses "Divine Rank":
The divine beings of the multiverse are often categorized according to their cosmic power. Some gods are worshiped on multiple worlds and have a different rank on each world, depending on their influence there.
It then goes on to discuss greater and lesser deities, quasi-deities, demigods, titans, and vestiges.
In the Players' Handbook, under Cleric is says:
Divine magic, as the name suggests, is the power of the gods, flowing from them into the world. Clerics are conduits for that power, manifesting it as miraculous effects. The gods don’t grant this power to everyone who seeks it, but only to those chosen to fulfill a high calling.
In a pantheon, every deity has influence over different aspects of mortal life and civilization, called a deity’s domain. All the domains over which a deity has influence are called the deity’s portfolio.
So we can determine that the default assumption is that there is a hierarchy of gods, that they have areas of influence, and that they grant powers to their clerics.
So that means that their powers aren't infinite, and they are in some level of conflict with each other, from cooperation to out-and-out war.
So they are giving some of their power, a limited resource, to their clerics.
Experience is needed to handle that much power
Since they don't grant power to just everyone, perhaps clerics need practice in order to build up to being able to handle the full power gods can give out.
If they are in competition or conflict with each other, if one god grants high levels of power to just anyone, then perhaps the gods they are competing with will too, leaving them poorer in power, but no better off. Better to carefully give out power, using their clerics as pawns to jockey for position.
Determining who is worthy
If power is a limited resource, the gods must need to determine who among their followers is worthy of more power.
It isn't just clerics
Why don't other classes start off at advanced levels?
Myth, legend, and fiction are full of adventurers who were mighty at a young age, well before their prime, granted their powers by the gods or fate; for instance, many heroes in Greek myth.
You don't have to play that way
You can start all your classes at whatever level you want.