They aren't in just one camp. They are moved to other camps away from the front lines to avoid them being freed by allied armies.
From the
wiki page on Stalag Luft III:
Just before midnight on 27 January 1945, with Soviet troops only 26 km (16 mi) away, the remaining 11,000 POWs were marched out of camp with the eventual destination of Spremberg. In freezing temperatures and 15 cm (6 in) of snow, 2,000 prisoners were assigned to clear the road ahead of the main group. After a 55 km (34 mi) march, the POWs arrived in Bad Muskau where they rested for 30 hours, before marching the remaining 26 km (16 mi) to Spremberg. On 31 January, the South Compound prisoners plus 200 men from the West Compound were sent by train to Stalag VII-A at Moosburg followed by the Centre compound prisoners on 7 February. Thirty-two prisoners escaped during the march to Moosburg but all were recaptured. The North, East and remaining West compound prisoners at Spremberg were sent either to Stalag XIII-D at Nuremberg on 2 February or to Marlag und Milag Nord at Westertimke.
With the approach of US forces on 13 April, the American prisoners at XIII-D were marched to Stalag VII-A. While the majority reached VII-A on 20 April, many had dropped out on the way with the German guards making no attempt to stop them. Built to hold 14,000 POWs, Stalag VII-A now held 130,000 from evacuated stalags with 500 living in barracks built for 200. Some chose to live in tents while others slept in air raid slit trenches. The US 14th Armored Division liberated the prisoners of VII-A on 29 April. Kenneth W. Simmons's book
Kriegie (1960) vividly describes the life of POWs in the American section of Stalag Luft III in the final months of the war, ending with the winter forced-march from the camp, ahead of the advancing Soviet troops and eventually being liberated.