Ksav Sofer explains that Yisro originally saw the long line of people who came to be judged by Moshe and thought that court was in session like this all the time. In truth, this was a one-time occurrence. Rashi tells us that this was the day after Yom Kippur, right after Moshe come down from Sinai, and there was a long backup of cases waiting. However, Moshe acknowledged that even without the backup of cases, there would still be a long line of people waiting, as they did not just come to him for judgment, but they came “lidrosh Elokim,” for brachos and tefilos and other help (see Ramban). Moshe Rabeinu was telling Yisro that he wasn’t just the Av Beis Din of the community – he was the Admo”r as well. If so, argued Yisro, all the more reason to get help. Even if you are as great as a Moshe Rabeinu, multitasking doesn’t work – you can’t wear multiple hats, do multiple tasks, divide your attention between different things at the same time, and accomplish them all well.
(Side point – the meforshei Rashi ask: Rashi 18:13 writes that it is impossible for court to have been in session all day, from morning to night. Chazal tell us that a judge who judges truthfully even for one hour gets credit as if he spent the whole day, morning to night, learning Torah. If so, why was Yisro so adamant that Moshe would wear himself out?)