[Fyrestorm3]

Quite simply, because the AI is not perfect and cannot think like a human being.

Here's what most likely happens in these kinds of situations: The AI team sees a highly-rated player that it wants to acquire, and does so. But when you come along offering a trade, the AI does not evaluate the recently-signed player any differently than the long-term players they also have. They're all just bits of data. Especially with international prospects, the scouted ratings of the player in question may have changed since he's been on the team. Or perhaps you've offered the team a player of a position that they majorly need. They could also look at it as a chance to dump payroll, if they're not trying to Win Now. And the Trade and AI Eval settings play a huge part in how the computer evaluates the worth of players in trades (which can be much different than in Free Agency).

Basically, the AI (by definition) does not have the ability to reason and think "Well, I've just signed this player to a massive contract; it would be silly to trade him away." It sees the player with the massive contract, and may value him less than the middling player with a manageable contract you're offering in return. Especially with scouting enabled, the ratings you see may not be the ratings the computer sees.

It's impossible for the AI to take all the same factors into consideration that a human would - otherwise it wouldn't be very Artificial. So sometimes, you have to police yourself if you want to maintain realism. If you don't care, then by all means go for it! But if it bothers you, then look into increasing the trading difficulty, or tweaking the Scouting and/or AI Eval settings.