Today, we have GPS and all sorts of tools for measuring distances and communication. For us, doing that is easy and I think that means we forgot that this was not always the case.

Artillery during World War One was a incredibly useful tool but it was not by any means an accurate one. The main problem was communication that was spotty at best. Coordinating the barrage proved to be almost impossible at the start of the war but developed over time with different results.

You must also remember that there was not just one trench - the defences was often multilayered. If one trench was about to be overrun, defenders could retreat to the second or third line and put the attackers under heavy fire from there. Due to the inaccuracy of the artillery pieces, you could never count on the barbed wire being destroyed or the enemy bunkers put out of action.

The best example of how the British tried to overcome this issue was by a rolling barrage - the barrage would cut off the attacked trench from support from the others. This did not always work as intended and the first line still possessed considerable firepower.

At the end of the war, it was considered that a rolling barrage followed by a quick strike often was more effective then weeks of bombardment.

Sources: "First Day at Somne" Chapter; Military Failures by David Saul.

but why didnt, in WW1 when they wanted to attack an enemy trench didn't they continue to bomb the enemy trench with artillery, meaning your enemy has to shelter underground, while your troops advance safely across no-mans land and then stop the artillery barrage just in time when they have safely reached the enemies trench.

This is essentially what was done; /u/JonathanRL talks about the rolling barrage/creeping bombardment, which the French utilized in mid-to-late 1915 during their offensives in Artois and Champagne, and which the Germans utilized on the Eastern Front at the same time. The British first used them on a large scale at the beginning of the Somme Campaign, where they experienced mixed success at first, but became a crucial factor in British and French sort-piece attacks in 1917. It wasn't until 1916 that the Germans and the French had sufficient guns and shells to carry out such complex fire plans with their huge expenditure of shells, while it wasn't until 1917 that the quality of British guns and shells was sufficient enough to utilize them comprehensively.

It's also important to note, as I do here, by 1917 the way the Germans organized their defenses had changed, becoming more fluid and dispersed, making the Artillery's job that much more important.