Why Must Extra Spatial Dimensions in String Theory Be Compactified?

[RobCS]

Hi, sorry if this is a daft question, but it's something I've often wondered about. why is it that the extra spatial dimensions in string theory have to be 'wrapped up' rather than extended like the 3 we experience? it seems to be that since we are 3D beings, 3 dimensions is the limitation of our perception - in the same way a 2D lifeform would have no perception of the 'Z' axis. Just because we don't experience more than 3 space dimensions doesn't mean that they're not there, does it?

 

[Gokul43201]

This is not about human ability to perceive dimensions. Physical laws reflect the dimensionality of the underlying space. Power laws seen in expressions for fields and currents would be different if the sources lived in greater or fewer (flat) dimensions.