This discussion brings up the very contentious issue of the value of Strategic bombing during WW2.
But I think several points already raised, and one that hasn't, explain why the Soviets didn't have much of a Strategic bombing force.
They knew the British, and then later the Americans as well were hitting the German Strategic targets already, better than the Soviets likely could have done even if they had put a lot of resources into it. They had major technical issues, such as with the AM-35 engines and shortages of things like aluminum, limiting their ability to meet the technical challenges. The Pe-8 was a good start for 1941, IMO, but there were major teething issues that didn't get resolved for years, and we know that Strategic bombing over Germany was no simple matter of sending heavy bombers over. The Germans made it very lethal for the British and Americans in spite of their advanced industry and resources. And planes like the Mosquito and the Mustang. And also as was pointed out upthread, the German Strategic targets were put out of range pretty quickly by the rapid advance of the German front line.
Which brings up what I think is the biggest issue is that the Soviets focused on Tactical and what I call "Operational" bombing instead of Strategic, for the very simple reason that they were facing an immediate, pressing, and ongoing threat from German ground forces which made the rapid destruction, curtailment and even just slowing down of the German land army the urgent priority over all other matters.
The US was really under no threat of a land invasion in their mainland during the war, even if battles overseas had all been catastrophic failures. The British were at somewhat more of a threat but everyone knew the Germans would have to get through the Royal Navy to land on British shores and that was a tough nut to crack. Commonwealth nations were in a similar, relatively safe situation. The Soviets however were looking down the barrels of panzers from day one and the threat was all too real. Real enough that they had to move most of their key strategic industries across the Urals and several promising weapon systems were badly delayed or just finished off by early German Strategic bombing conducted with basically light bombers.
By "Operational bombing" I mean targets which are in between tactical and strategic: concentrations of supplies put into position before major offensive or defensive operations; the destruction of bridges, railyards, docks, and other transportation infrastructure (such as we have recently seen the Ukranians do so masterfully in the current war) and so on. The Russians did hit targets like that, as best they could, using Pe-2s, Il-4s and later US made B-25s.
But they increasingly had to conduct these raids at night, which as the British learned, meant low accuracy, and over time, vulnerability to night fighters. The Soviets didn't have large numbers of long range escorts (the rather flawed Yak-9DD came later in the war and not in very large numbers) and I don't think (?) they really had any night fighters to speak of except maybe a few Pe-3s. So as the difficulty of the longer-ranged night bombing campaign ramped up, the payoff in terms of resources expended to damage done decreased.
Besides, after Stalingrad, they were mostly winning, so if "it" (their emphasis on Tactical bombing) ain't broke, why fix it? We can debate the effectiveness of Il-2, Pe-2, and fighter bombers in the Tactical role, but the Soviet system was clearly working by 1943, and was becoming dominant by 1944, albeit at great cost in casualties and resources.