Actually troops were evacuated from ports like Danzig (Now known as Gdańsk, Poland) and Gotenhafen (now known as Gdynia, Poland) to mainland Germany. The Kriegsmarine (Navy) did have control of much of the Baltic Sea. Civilians were also evacuated from these port cities, but often at a very heavy cost. Many troops were pulled back from the Vistula area, and evacuated through the aforementioned harbors in those port cities.

Many German troops were evacuated from Kurland Kessel (The Courland Pocket) via the Latvian city of Ventspils, or via the long and treacherous way south westward across the Frische Nehrung ( Fresh Spit. Now known as the Vistula Spit, and is the part of Russia that used to be part of East Prussia) and or at times the semi - frozen Frische Haff (Fresh Lagoon. Now known as the Vistula Lagoon. It was partially frozen in places in the winter of 1944 / 1945). Many civilians fled with the troops / towards where many German divisions were pulling back to, either going to Courland or trying to escape East Prussia. Most of these civilians were from the East Prussian capital city / region of Königsberg (now known as Kaliningrad, Russia) tried escaping Northward towards the Frische Haff and Frisched Nehrung in hopes of being rescued by the Kriegsmarine and ferried to mainland Germany (with the protection of German military units). Many German troops were evacuated, but many remained in Courland, and some were even ferried to Courland (thinking instead they would be ferried to Danzig, or Gotenhafen and from there catch a larger ship to mainland German ports like Lübeck). However, there were two main reasons German troops were kept, and at points even funneled in to the Courland Pocket.

The first and main reason was so that Germans in far NE East Prussia and Volksdeutsche (Ethnic Germans living outside of Germany) from lands that were former Latvia (and now are part of Latvia) could escape to East Prussia (or later escape via the ports in Courland), and from there to Germany proper. So much of the reasoning behind leaving troops behind in Courland was to protect those territories so German civilians could escape the Soviets. Similar to Courland, the Frische Nehrung was held bitterly in to 1945, manly so German civilians could flee advancing Soviet units and be ferried to Germany proper.

The second, and main non humanitarian reason for German troops staying behind in Courland, and not being pulled back to fight on other far more important fronts (like the crumbling Vistula - Oder Front) was because Hitler personally wanted them there! He wanted the Courland Pocket to hold out at all costs so he could use them as a possible future springboard from whence a counter offensive against the Soviet Union could be launched. Of course this was crazy, and utterly foolish thinking. But the German soldiers in the Courland Pocket followed their orders, and did not surrender to the Soviets until May 9-10, 1945. They were the last major German hold out to surrender.

Now the Soviets for their part largely bypassed the Courland Pocket. They could have easily pushed the Germans back in to the Baltic Sea (and either surrender or die), like they did in Danzig, Gotenhofen, and on the Frische Nehrung during their assaults in East Prussia - but they did not. Similarly they did the same thing further west and south when bypassing / besieging the German Silesian city of Breslau (now Wrocław, in western Poland). Sure the German defenders fought bitterly against their larger Soviet foes, but in reality the Soviet Red Army was pulling a page out of Germany's military playbook and bypassing hard targets that were well dug in, but could be dealt with later. The Soviets had bigger fish to fry, and that was Berlin. So places like Courland, and Breslau were of secondary concern, and ancillary objectives. The main prize was Berlin, and the Soviet Red Army steamroll was hellbent on crushing the Nazi's capital city - even if it meant leaving a few German divisions as a small thorn in their side, up in Courland.

By the winter of 1944 / 1945 the Red Navy did have control of the Eastern parts of the Baltic Sea, and the Germans paid dearly for their mistakes of sending navel forces elsewhere in the war. The German military, with the Kriegsmarine leading the effort, commenced Operation Hannibal which was the evacuation of German civilians and military personnel from East Prussia and the Baltic States - think of this as the German equivalent of the Dunkirk evacuations. Many German civilians and military personnel escaped, but many did not. The largest maritime disasters in all of human history occurred during Operation Hannibal, in the Baltic Sea in the final months of World War II, when Soviet submarines sunk German ships predominantly carrying East Prussian and Volksdeutsch refugees (but also some military personnel); these ships that were sunk were the MV Wilhelm Gustloff in January 1945, (killing a total of between 9,000 and 10,500 people - mostly civilians), MV Goya in April 1945 (killing between 6,000 and 7,500), and the SS General von Steuben (killing 4,500 people, with about half of that figure being evacuated wounded soldiers) in February, 1945.

So to TL;DR the Courland Pocket was kept in place as a way to put pressure on the Soviets in the Baltic / far East Prussia, so that German civilians and other military personnel could escape to Germany proper. And it was also left there in case Hitler pulled a miracle for crumbling Germany out of thin air, turned the tide against the Soviet Union, and pushed them back - Courland would be a springboard to launch future offenses in to the Soviet Union. Of course this was not a feasible reality, and many German soldiers that were needed elsewhere were wasted in holding the Courland Pocket.

• "Battleground Prussia: The Assault on Germany's Eastern Front 1944–45" by Prit Buttar (this is a thick and dry read at times, but covers the topic very well)

• "In Deadly Combat: A German Soldier's Memoir of the Eastern Front" by Gottlob Herbert Bidermann

(Bidermann's personal account of fighting on the Eastern Front, and being cutoff in the Courland Pocket at the end of the war is a fascinating and must read, if you are interested in a more personal account of the battle)