You can find endless people who have never had a card failure, and endless people who have had 1 or more card failure.
It's a risk not worth taking for paid professionals. Simple as that.
And you can find endless people who have had a card failure, that have had no sizable effect on payment to said professionals.
Two slots is not the only solution for redundancy in the field. Conflating the risk with one specific solution does not lead to a valid conclusion.
It is an impossible to replicate component of redundancy. You cannot get a simultaneous write at time of capture to another bit of media any other way right now. A second card slot is the only current solution that provides that for any ilc, anything close is much slower, and moreover requires external devices. And why anyone would vehemently reject the capability is baffling. A second card slot is cheap to include and easy to use. Why people doing paid work don't simply demand it is crazy. It's akin to not backing up to another hard drive or to the cloud. Just complete and total reliance on a single point of failure at a step of the image pipeline. Who thinks that is the perfect and optimal way to do things? If it was prohibitively expensive, or complicated, or relied on unknown componentry, then maybe. But, it isn't a single one of those things. It's so dead simple that the aversion to it is mind numbing.
No, there are wireless synching solutions--and an external storage device provides more reliable recovery, in case the primary device is lost or stolen.
Wireless solutions would be akin to backing up to another hard drive or to the cloud. Backing up to another hard drive or to the cloud requires external devices.
If you are RAW shooter - how long will you have to wait for the transfer of a single file?
My experience with on the fly use of external storage media: forget about it!
Not every photo session bears the risk of being not repeatable.
But if you have a situation you cant repeat, the security of two storage media is important.
I did some photographic work that has to be done at a certain day. On the same day we would have the chance to repeat the shooting of single photos - maybe even the entire series - but if a mistake was noticed the following date it would have been a drama.
First I did this shooting with Pentax K5. At the end of the session I transferred the data to a notebook and did a check of the files on the notebook in terms of technical quality and completeness.
Then we bought K3 which had two slots. This was a great improvement as I could do the check within the camera and even if one card should be corrupted the second would have the entire set of data.