Can we try to make a distinction between "easier" and "simpler"? Simple vs. complicated rather than easy vs. hard. Many bass parts are simpler than many guitar parts. That is the nature of the role of the bass in creating a particular ensemble sound. Alternating roots and fifths with an occasional scale up or down between chords on the bass is simpler than playing a lead solo on guitar, but that is the nature of bluegrass and many other country music styles. Adam Clayton playing steady eighth note roots has a simpler task than the other members of U2. That does not make him a bad bass player. He has a role to play and he does it very well. His role is to supply the pulse and the harmonic foundation. There are also simple guitar roles, such as strumming rhythms. There are also less simple bass roles, such as the role James Jamerson created for himself at Motown. Still, even Jamerson used simpler parts for certain types of songs, such as most of those by the Supremes.
It might be true that it is easier to play most bass parts than it is to play most guitar parts, since it is generally easier to play simpler parts than more complicated parts. This has nothing to do with how many strings each instrument has and everything to do with the role of the instrument within particular types of musical ensembles. It means that a bass player may reach a certain level of competence faster than a guitar player, because he is called on to play simpler parts. Mastery of the simple role of the bass is not so easy, however. Mastery of this role requires confidence in the harmonic changes, absolute accuracy and fine control of timing, as well as careful listening to the other members of the ensemble.
It might be true that it is easier to play most bass parts than it is to play most guitar parts, since it is generally easier to play simpler parts than more complicated parts. This has nothing to do with how many strings each instrument has and everything to do with the role of the instrument within particular types of musical ensembles. It means that a bass player may reach a certain level of competence faster than a guitar player, because he is called on to play simpler parts. Mastery of the simple role of the bass is not so easy, however. Mastery of this role requires confidence in the harmonic changes, absolute accuracy and fine control of timing, as well as careful listening to the other members of the ensemble.