You shall not build an altar of HEWN stones

[tonychanyt]

Why not?

Let's see the context, Exodus 20:

22 the LORD said to Moses, “Thus you shall say to the people of Israel: ‘You have seen for yourselves that I have talked with you from heaven. 23You shall not make gods of silver to be with me, nor shall you make for yourselves gods of gold.

This is the context: making idols.

24 An altar of earth you shall make for me and sacrifice on it your burnt offerings and your peace offerings, your sheep and your oxen. In every place where I cause my name to be remembered I will come to you and bless you. 25If you make me an altar of stone, you shall not build it of hewn stones, for if you wield your tool on it you profane it.

Ellicott's Commentary for English Readers:

(24) An altar of earth thou shalt make unto me.—The earliest altars were, naturally, either of earth, or of unhewn stones, gathered into a heap, since these could be constructed with little labour, and without tools. But, as civilisation advanced, more elaborate structures took the place of the primitive ones. It became usual to erect altars of hewn stone, adorned with carvings more or less rich, among which might often be introduced human and animal forms. We must understand the command here given, and that of Exodus 20:25, as intended to forbid structures of this latter kind, which, if allowed, might have led on to idolatry.

Why not build an altar of hewn stones?

To avoid any possibility of idolatry associated with man-made objects.

Later Solomon built the temple with hewn/dressed stones in 1 Kings 6:

7 In building the temple, only blocks dressed at the quarry were used, and no hammer, chisel or any other iron tool was heard at the temple site while it was being built.

That's the temple, not the altar itself.