The reasons why parachutes weren't worn are unclear, even to those who flew in WW1.
Harold E. Hartney, for example, who served in both the RFC and US Air Service, later wrote in his memoirs,
Up and At 'Em (near the beginning of Chapter 11), "Why our fliers didn't have them [parachutes] during the war I shall never know . . . In the World War it would have saved us scores and scores of our aviators' lives." (In Chapter 14 of the same book though, somewhat inconsistently, Hartney opines that the use of parachutes could result in some pilots bailing out too readily and states that pilots felt it was "sissy" to wear 'chutes.) Arthur S. Gould Lee, MC, who flew in the RFC/RAF, wrote in one of his books,
No Parachute (at the beginning of Appendix C), that "Nothing more mystified the pilots and observers of the RFC, RNAS and RAF during 1917-18 than the dour refusal of their High Command to provide them with parachutes."
The best study on this question of parachutes that I've ever seen, is one done by the above-cited
Arthur Gould Lee. Driven by his deep personal interest in the topic, Lee, who became a career RAF officer after the war and rose to the rank of Air Vice-Marshal, closely examined all War Office files from 1914-18 relating to parachutes. His account of his findings and conclusions appear in Appendix C ("Why No Parachutes?") at the end of his book
No Parachute.
His work is too long to repeat here in detail, but here's a very brief summary of his conclusions:
1) Promising parachute designs existed even before the war, but during the war little thought was given by anybody in authority to developing or adapting them for general use in airplanes. It is true that the primitive airplane of 1915-16 could not have carried the weight of a parachute without some sacrifice of performance, but this wasn't true of airplanes in 1917-18. A practical and reliable parachute could have been developed within a few months, by refining existing designs.
2) Some British officers claimed that fliers didn't wish to carry parachutes. But these were senior RFC and RNAS officers "who had never experienced fierce lethal air combat, nor witnessed the desperate need of comrades falling to their death in broken or burning planes". Those in the fighting ranks felt differently. "[T]he active-service flier did not want to wait for perfection. He wanted something quickly, that would offer a hope of escape . . . Even if the parachute were not infallible it would offer a sporting chance, which was better than a death that was certain."
3) Concern that parachutes might encourage fliers to abandon their aircraft too quickly and without a determined fight is an "invention" and was NOT an official attitude. Lee's own view too was that, from early 1917, "there were few fliers with any experience of air fighting who were not obsessed to some degree, though usually secretly, with the thought of being shot down in flames . . . To all these a parachute would have been a stiffener to strain and morale."
4) General Trenchard cannot be blamed for the official bias against parachutes, as he sometimes has been years afterward. During the early and middle stages of the war, Trenchard was not the dominating figure he later became. He simply did not have the influence, through much of the war, to impose his views on officers of equal or higher rank and to single-handedly scupper the use of parachutes (even if he had wanted to). Also, as one who was a commander in the field through most of the war, he would've been "much too preoccupied with weighty day-to-day problems of high direction to be able to intervene frequently in this one question of technical equipment."
Lee's bottom line conclusion:
For the failure to introduce parachutes into the RFC/RAF in 1917-18, "no one man, nor even any specific group of men, can in fairness be indicted. It was the collective official mind actuated by intangible prejudice which was responsible. As Calthrop wrote with understandable bitterness in January 1919: 'No one in high quarters had any time to devote to investigating the merits of an appliance whose purpose was so ridiculously irrelevant to war as the saving of life in the air.' "