Without claiming to know much, if anything, about accounting for player transfers, I suspect it's related to the fact that his market value is declining faster than his salary. His impact on the income statement is not declining, yet his value as an asset on the balance sheet is - at a faster and faster rate.
To renew the contract, we would presumably have to accept an increase in our liabilities to the value of his future wages (or whatever the contract stipulates to be the cancellation amount) - balanced by an equal increase in assets. However given his age, this asset would now have to be depreciated faster than previously over the term of the contract. Faster depreciation -> drop in income -> squeeze on the bottom line.
Of course, his contribution to our revenue (e.g. Champs League qualification) is literally immeasurable. As are the facts that he is an outstanding ambassador for the club, and has so much experience to impart - how do you value these intangibles?
Therin lies the problem - we have a bunch of suits in the board who know nothing about football and everything about sales, marketing and finance. To them, the only 'value' players have are the tangibles on the financial statements. They have no respect for the intangible value that legends like Lampard add to the club and what they mean to its fans and its history.
Lampard is undoutedly Chelsea's greatest ever player (yes, more so than Zola and Osgood), but in the world of Buck, Gourlay & Emenalo, there are no intangibles when it comes to playing assets. In short, they have no idea what club football is all about.