Why are credit card transaction fees often around $0.25 or $0.35 per transaction?

This is assessment fees. It is applied by card associations when you use their network. Assessment fees vary world over. Some countries don't even use the nomenclature assessment.

In many countries in Asia for example, the flat assessment fees is not included, it is part of the overall interchange.

Merchant Mavericks has a really great description on this:

Flat Fees
The below mentioned flat fees all fall under the markup category. These fees can be negotiated all day.

They may vary by name, value, and applicability, but at least some of them will probably show up on your credit card processing statements each month. It is your job as a merchant to understand them and dispute any fees you feel unnecessary to pay.

• Terminal Fees: These are charged to merchants who have physical stores, where they directly swipe a customer’s card. If you run a business online, you will not have to worry about this. Some providers try to lock merchants into terminal leases, but as we’ve mentioned before, don’t lease a terminal. Most of our favorite providers will encourage you to buy your machine outright for a low one-time fee. This can save literally thousands of dollars in the long-run. For an example of this, check outHelcim.
• Payment Gateway Fees: These are similar to terminal fees, but they are applied to ecommerce businesses instead. Some processors have in-house payment gateways that are free of charge (CDGcommerce). You can learn more about payment gateway’s here.
• PCI Fees: These are fees paid to the Payment Card Industry, either for noncompliance or compliance. In the case of noncompliance, you have to pay because your business is not upholding PCI standards, which could cost you even more money in the long run. In the case of compliance, you have to pay the merchant account provider to make sure you remain in line with the regulations at all times. Unfortunately, some providers charge for this service without actually providing it, so you need to make sure you are being cared for at all times.
• Annual Fees: These are fees charged every year to cover the basic use of a provider’s services. In my opinion, this is a bogus fee. Most of the better merchant account providers will not charge it.
• Early Termination Fees: This is pretty self-explanatory. It is a fee that is charged if you cancel your contract early. Another fee you definitely want to avoid.
• Monthly Fees: These are fees that are charged each month, usually for the purpose of covering call center costs. Ironically, most of the phone calls that come in are the result of mistakes made by the merchant account providers, making them the cause of their own fees. If you’re looking for the lowest monthly fee possible (a good idea if you have a low volume) take a look at Payline Data. They have a plan for just $5 per month.
• Minimum Fees: These are fees charged to merchants who do not reach a certain transaction total for the month or year. The minimums will vary by provider, but most of them are around $50,000 a year. This is another fee that is not charged by some of the better providers like Dharma Merchant Services.
• Statement Fees: These are fees charged to cover printing and mailing costs for credit card statements. Some merchants bypass these costs by using electronic bill statements, but others pay as much as $15 a month for miscellaneous processing costs.
• IRS Report Fees: These are fees that merchant account providers charge in exchange for reporting transaction information to the IRS (1099-K). Most of these charges range from $2 to $5, depending on the provider.
• Online Reporting: These are alternatives to statement fees, charged to merchants who choose to view their statements online. Most providers will not charge this kind of fee, and those that do often lump it together with others.
• Network Fees: The card networks charge certain non-negotiable fees that are passed through to the merchant, such as the FANF.

Source: The Complete Guide to Credit Card Processing Fees & Rates

Another resources to understand assessment fees:

• Credit Card Processing Fees
  
• Interchange Fees
  
• What are “Credit Card Assessment Fees”?