Because he has stopped negotiating with himself, and he is simply putting forward his favored solution to Republicans, and demanding proposals from them:

Amid demands from Republicans that President Obama propose detailed new spending cuts to avert the year-end fiscal crisis, his answer boils down to this: you first.

Mr. Obama, scarred by failed negotiations in his first term and emboldened by a clear if close election to a second, has emerged as a different kind of negotiator in the past week or two, sticking to the liberal line and frustrating Republicans on the other side of the bargaining table.

Disciplined and unyielding, he argues for raising taxes on the wealthy while offering nothing new to rein in spending and overhaul entitlement programs beyond what was on the table last year. Until Republicans offer their own new plan, Mr. Obama will not alter his. In effect, he is trying to leverage what he claims as an election mandate to force Republicans to take ownership of the difficult choices ahead.

His approach is born of painful experience. In his first four years in office, Mr. Obama has repeatedly offered what he considered compromises on stimulus spending, health care and deficit reduction to Republicans, who either rejected them as inadequate or pocketed them and insisted on more. Republicans argued that Mr. Obama never made serious efforts at compromise and instead lectured them about what they ought to want rather than listening to what they did want.

So the Republicans have been shocked, that gambling is going on he’s decided to stop conceding at the start of talks.

BTW, the Republicans have now to been forced to make a real in response, only it’s completely number and specifics free. Yes, the propose raising medicare age to 67, which won’t save any money, because the 65-67 year old’s will need insurance subsidies, unspecified loophole closing (gee, they don’t want to be the first ones to propose cutting the mortgage deduction?):

The offer makes no mention of how to handle a looming battle over the federal debt limit, which will soon need to raised above $16.4 trillion. Nor does the offer provide any suggestions for how to carry out the debt-reduction framework and avoid the fiscal cliff. GOP aides said those details are open to negotiation.

“There are multiple ways to get this done,” said one senior GOP aide who briefed reporters on the condition of anonymity. “The details need to be filled in later.”

(emphasis mine)

The Republicans want to make the Democrats propose specific cuts, and then bludgeon them them with that in 2014.

Do not expect the GOP to bargain in good faith, they are hostage takers, and will be for the foreseeable future.