You can come up with many reasons for it.
Just like you could come up with many explanations for what you are experiencing and misconstruing as a "handicap". Don't forget I used to defend this position. And to an extent sometimes I find myself thinking it's there. But then logic takes over.
The evidence for handicap from different people who describe it is often self contradicting. People will say "it helps to have a low rated player" others will say "it helps to have a high rated player" etc etc.
Think about it this way. What is more likely. EA actually implemented a system like this and found a way to keep their 100s of employees in check somehow (not leaking this valuable information). Not to mention somehow testing different combinations to find which handicap produces the best pack purchases. Because you may very well create less demand for better cards by leveling out the competition.
They would have to do some intensive testing. Which again requires more employees which again is more people they would have to get some Fifa coins. OR In reality what you perceive as "handicap" is simply parts of code that you don't understand. There is tons of behavior for the AI that we simply don't get. Because we didn't program it. It works well with your examination. Think about it.
What you say definitely makes a lot of sense, but idk haha I honestly wouldn't put it past a huge corporation like EA to put the effort into it. Again, this game has been around for 20 years -- they've had more than enough time throughout the years of various releases to hone their methods and practices.
When all is said and done though, all I have to go off is my personal experience, and based on that I believe that it does really exist.
For something like this it would take 1 whisteblower. Considering the consequences..... you really think it would be worth it for them?
Basically "Guess what.... getting better cards is pointless because we decide who wins anyway... and I have proof because I was in that project, here are some emails I saved".
That alone is enough to discredit this theory in my eyes to be honest. No company would want that liability.