It's not the customer's fault but if it's out of stock and can't be reserved, it's not the salesperson's fault either. Working in the same company means nothing, people in shops just don't have the authorisation to do something like that. I don't know about other shops, but we are given the option to have an item sent out to the customer/store, provided we have some in stock at our warehouse. I'm sure you can imagine however, if an Item is popular enough, there may be none left in the warehouse either.
As for reserving items in another store, reserving them automatically is simply impractical and just could potentially just p*** more customers off. How would you feel if you were about to buy something in a shop, had been through the specs, a demonstration and decided this was the right one for you, only to find out you couldn't have it because someone a mile away had just reserved the last one.
An automatic reservation system also has no idea about the condition of the item. Often, parts may be taken from boxes in the stock room because of customer care issues etc. It would annoy you even more to drive to another shop or to come back to the same shop the next day, only to find out the laptop that was delivered for you, had no charger or manuals.
Often, because of these limitations, the only practical way to reserve something at another store, is to phone them and have them wrap a piece of paper around it with your name on. If that store is busy (as most are around this time of year) then nobody is going to be able to answer the phone and thus, your item is not going to be reserved.
If there is no way to get reserve the item automatically and the other store is not able to answer the phone, what good is calling the manager going to be?
If all of the options available to them have been exhausted, then you can either pick something else similar, or take the risk of going to the other store. As long as the customer's expectations are managed correctly, it's reasonable to suggest this. It's not reasonable to use it as an excuse to have an argument with the salesperson, it's not going to get you anywhere and will just risk being refused service.
Also, if they are telling you their system doesn't let them do something, it usually means they are either not allowed to do it, or it's simply a not a service the company offers