Combination of factors.

Each unit whether it’s a tiny coin or a bullion bar has to go through multiple assays to confirm legitimacy so you can imagine the same weight in coins will take a lot longer to verify than a single bar - those costs in time/labour add up and get spread across all the units.

But then it’s also a factor of sellers not wanting to deal with small volumes. If you can make $1000 selling one bar or $1 selling one coin (just as an example) then you’re going to spend a lot more time/labour selling enough coins to make up the same profit margin as a single bar sale. It’s almost an “annoyance cost”; they’ll sell it to you but it’s a pain in the arse for them to deal with more customers and make less money so they jack up margins to compensate.

The sweet spot for them is likely those 1/2oz or 1oz coins in terms of unit profits - selling bars would be less frequent but you’d rather make up for that with a few coins than with hundreds.