If the universe is flat (which measurements back up), then the universe is not curved onto itself. So, the ad hoc explanation of inflationary theory is added.
In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10−36 seconds after the conjectured Big Bang singularity to some time between 10−33 and 10−32 seconds after the singularity.
Note this is entirely theoretical and there is no empirical evidence to support this. So why would cosmologists continually propose ideas that are ad hoc and have no empirical evidence to support it?
I believe the primary reason is the assumption of naturalism in science. Ironically, they must invoke extranatural explanations (curved wrapped universe, inflation) that have no evidence to support them. As we look at areas dealing with cosmology, we see the cracks of the assumption of naturalism leading to beliefs that lack any evidentiary support.