Because the French where almost finished and the Wehrmacht was spred deep inside french territory, so all of his military commanders might be wrong and would be no more a long war.
Putting aside for a moment the fact that Stalin's generals were telling him that this would be a long war because that's
what Stalin himself believed (generals who said otherwise found themselves with very shortened careers, not to mention lives), there is the fact that the German Wehrmacht had just demonstrated what could be done with mechanised mobile warfare; a form of warfare that the Red Army had no capacity for because Stalin had just had its principal proponents shot, along with 33,000 other officers.
He would also know that his forces wouldn't get very far before the Germans were able to redeploy forces from France to their Eastern Front; the Red Army was designed along the same lines as the French Army, based around massed infantry with armour in support - which the Germans had just demonstrated a superiority to.
Then there is the problem he would be facing trying to sell this war to the people; the Tsarist regime had collapsed because of a reclass military adventure; there was a deep seated fear amongst the Bolsheviks of rebellion if they became involved in a protracted war and a very deep suspicion as to the real loyalty of the Red Army; hence the purges. They'd only just sold the country on the pact with the Fascists, to then turn around and attack Germany would have undermined all of that propaganda.
With France out of the picture, GBR might also ask for peace.
Far from just fearing that the British were about to accept terms from Hitler, Stalin had considerable concerns that the British could throw in their lot
with Germany. On the eve of the German offensive the British replaced Chamberlain as prime minister with the most reactionary man in the House of Commons; as Home Secretary Churchill had been a notorious opponent of organised labour, then as Secretary of State for War he had sent troops and munitions to Russia in an effort to crush with Revolution in its infancy, and he had spent the last twenty years in an unrelenting propaganda war against the Bolsheviks. That, combined with Britain's ongoing plans to bomb Baku which Stalin was kept fully informed of, and Stalin had fair reason to fear that he might soon be fighting the British; hardly an opportune time for risky adventures, and Stalin wasn't a man who liked taking risks.
Putting all that aside, then there's the not inconsiderable problem of incompatible rail networks to deal with after crossing the old Polish border, and then of course, there's the weather.