They did...

Part of the entire point of the German plans at Jutland were to draw the Grand Fleet over prepositioned U-Boats both at their ports and to ambush them as they came out in response to a German fleet sortie. Nearly 2 dozen U-Boats were deployed for the operation with most sent to patrol off Scapa Flow, with others to the Firth of Forth, and other ports and sea lanes the British would take. Several were also sent to lay mines to catch British ships by surprise! However problems arose with the timing of the operation forcing delays. The High Seas Fleet would not sortie until the battlecruiser Seydlitz was ready and out of the yards, that she took longer than expected pushed the subs to the limit of their endurance on patrol and several were out of position, plagued by bad weather, or had been sunk.

There is also the simple problem of hitting a warship on the open sea with a U-Boat. At best on the surface they could keep up with only the slowest warships, even the big dreadnoughts were faster than most subs doing over 20 knots in a pinch. When submerged they could barely keep up with anything being limited to 10 knots on a good day. That meant they basically had to already be in position to fire when the British warships came into the killzone. An extremely tricky proposition on the high seas, and one that rarely worked.

That is not to say that U-Boats didn't have success against Entente warships. A single U-Boat, U-9, in late September 1914 met with and sank 3 of the 4 old armored cruisers he encountered in the North Sea on patrol. While also in 1914 the outdated battleship HMS Formidable was also torpedoed and sunk while the admiral in charge took few precautions from subs.

But in the end the Germans saw that the subs as they existed were not really up to the challenge of successfully eroding the Grand Fleet or other British formations. Basic precautions like escort screens, booms and nets, zig zagging, maintaining appropriate speed, and harbor defenses all made it so that the U-Boats were far better positioned to attack merchant shipping with their slow speed and fragile nature than other combatant craft.

It basically went in waves, 1914 saw a mad rush to try to sink British warships with some moderate success but it didn't pan out. 1915 saw the first shift towards merchant shipping, but this was reeled in after protests from other nations, 1916 saw the second effort at a conventional victory and Jutland, then 1917 on the full on unrestricted submarine campaign. But it did impact British planning, Jellicoe and Beatty continually worried about the security of their bases from subs, and at times even removed the fleet to the West Coast of Scotland and bases on the Irish Sea for added security until Scapa and Rosyth could be made secure to their likings.