Why not a common public transport card?

We have credit and debit cards issued by Visa, MasterCard that works around the world. We don't get one card for each city we visit. 

We have sim cards that can work across the globe. Your airtel sim in India can be activated for international roaming, sim connects with a specific local mobile operator and you can use it abroad without having to buy a new local sim every time. (At times local sim is cheaper though)

However public transport is still very localized business. Each city has its own systems, technology, rules. 

Above: public transport cards from various countries I have visited.

Forget between countries, even within a state or province there is no common card. For example, in USA, California state:

• San Francisco has Clipper card
• San Diego has something else, PRONTO
• Los Angeles has TAP card

But this is lots of inconvenience for travelers.

• Need to buy new card for each city they visit
• Money left unused in a card often goes waste-in some places it is possible to get it back but time and effort involved is huge so people often leave it as it is
• Lots of cards get accumulated over time- no city names on them, need to remember which one is for which city, how much money is left and remember to carry them without fail before next visit.

A unified or common card is very convenient to users but probably brings in following challenges

• Will need a central body like Visa/MasterCard to facilitate exchange/settlement between different cities/public transport system and these guys will ask for a cut on every usage, increasing cost for consumers. Even if this body charges a small amount- say 10 paise per ticket, it adds up to lakhs of rupees every month- (Bengaluru has over a lakh people using buses n metro everyday, 60 lakhs per month is 6 lakhs in fees)- extra expense for the transport body.
  
• Public transport is mainly used by city's residents, tourists often prefer tour bus/hop on hop off etc, so the need for common card is only for a small % of budget travelers who prefer to use public transport instead of taxi or tour buses. 
• There is no extra money to be made by bringing in this new convenience to small portion of people. Because of this, a common card is not a priority for any city administration
• Will have to maintain a separate customer care to address issues and complaints- more cost
  
• Lack of standardized rules also hamper a common implementation
• Some cities we need to both tap-in and tap-out and money is deducted based on distance traveled, while other cities charge per each ride irrespective of distance
• Day passes, senior citizen discounts, minimum balance and other rules are not common across the globe
• Vending machines & POS units used by staff that issue the public transport card & top up may need a massive upgrade to allow common cards

The technology behind most cards is fairly common. An RFID based identification that the card is genuine and a mechanism to deduct certain amount each time card is tapped. So though there are complexities, it might be good idea if authorities try to standardize the cards within a county & state to begin with and then see how to make it standard across the world. 

Within a country it shouldn't be too complex- like can't I use same card for Delhi metro, Bengaluru Metro & Mumbai metro? How hard it is to bring in a solution that calculates how many Delhi metro cards are used in Mumbai and transfer corresponding money to Delhi Metro?

What do you think?