There are several aspects to this.

First of all, the Entente did try, on several occasions, but on a strategical level rather than a tactical level.

The Entente did land troops at Gallipoli in an attempt to knock the Ottoman Empire out of the war. These troops were partially used in the Salonika landing tro aid Serbia and possibly push Bulgaria or the Ottoman Empire out of the war.

The British also landed in what is tofday Kuwait and advanced through Mesopotamia, before being surrounded and forced to surrender at Kut.

Those attempts all ended in failure, often because the defender could, due to railroads, bring reinforcements and supply to the new front more quickly than the attacker (who had to bring them in by ship and then by small boats if he did not control a port that could dock a ship).

During ww1, the mine had reached maturity as a naval weapon, and it was extensively used, especially in shallow waters such as the Baltic Sea, the Danish straights and the waters outside Flanders.

There are still around 30 000 mines left in the Baltic Sea from ww1 and ww2, for example - most sunk at their anchor, but every spring there are a few that drift ashore after tearing their anchor chain during ice movement and they always cause quite the scare.

So, any British landing in Flanders would face extensive mine fields and would have to supply and reinfoce those forces by small boats from larger ships - a very inefficient, laborous and above all dangrous (since the ship needs to remain stationary and is an easy prey to submarines or fast torpedo boats) process. In the meantime, the defending forces can contain the beachhead and count on supply and reinforcements brought in by railroad.

Landings were done by rowboat in ww1, which was far less effective than the landing ships used in ww2 - they were open, unprotected and could carry far fewer men in each go. A hit on a rower could also make it unmanouvrable until he was replaced. It was especially hard to bring in support weapons (light artillery, trench mortars, infantry guns, heavy machine guns and so on) this way. This means the defender will have a substantial advantage in artillery. Naval support gunfire could be available, but not to the level of ww2, since there would beno radios to equip forward observers with to communicate with the ships and no spotter planes which would remain over the battlefield to guide the naval gunfire.

As for the Danish straights, they would run into massive mine fields that would require sweeping, as well as violating Swedish or Danish neutrality. The German Hochseeflotte could also use the Kiel canal to be either in the Baltic or in the North Sea, so you will need to fight it as well as force the minefield and protect any kind of landing.

Simply put - the technology and support needed for a successful naval landing did not exist in ww1.

Asked and answered here:

https://www.reddit.com/r/AskHistorians/comments/2imkx0/was_an_amphibious_landing_in_the_style_of/

There was a serious plan - Operation Hush - to land a division around Westende to turn the flank of the German lines in conjunction with Third Ypres. Special shallow-draught barges were built to carry a brigade each, including tanks and artillery, in conjunction with two monitors that would also provide propulsion and fire support. The land offensive didn't make enough progress to allow the landing force to link up, so the operation was cancelled less than 48 hours before it was to have commenced.