Plenty of people believe monopoles were created in the big bang!
However these are often features of GUTs so it is not expected to see them created in particle accelerators.
Also, since the existence of magnetic monopoles gives us charge quantization (which we do see of course), there are theoretical niceties they provide. There's also the famed "valentine monopole", where a monopole search saw one. It is always an interesting topic for discussing the scientific process, since if something is really rare ... how many do you need to see before you believe they exist? The layman will usually say one. The scientist of course wants more :)
That's putting the cart before the horse...the real issue is whether magnetic monopoles exist or even could exist...then we can dither endlessly about necessary and sufficient initial experimental conditions to produce and possibly detect them.
The immense majority of HEP theorists believe in their existence in one guise or another. They show up ubiquitously in GUTs, topological theories, various field theories, as well as in nonperturbative physics and quantum gravity.
So while you can contrive scenarios where they dont appear, lets just say they have a huge parameter space of possible places to come out off. The added bonus is they explain charge quantization in a really simple and elegant way and are a beautiful exposition of geometry in quantum theory.
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