What a horrible thing to say.
First off, at the most obvious and fundamental level, the statement is simply wrong. No instrument is easy.
But it's good to interpret the opposition as far in their favor as you can go. So I'll move down a level.
I think what most people actually mean to say, when they say this, is 'Bass is easier.' Easier than what? Easier than the little guitar with six strings.
So let's evaluate that interpretation instead.
It's not entirely untrue. There are senses in which it's easier. Almost all bass players (sorry Lemmy) play one note at a time, almost all the time. Six blades often play two or more notes at once.
But does that really make it easier? I would argue not necessarily. Depending on where the two notes are located, etc. But perhaps on a general statistical level it makes things a little easier, considered in isolation. And what about lead playing, supposedly the hardest part? That's single notes, like the bass, only with freer time, no?
A better argument, though, and one that I suspect many people with no musical experience actually rely on in making the judgement, is simply that there are fewer strings. And that's one case where the novice is right and the mid-level student often wrong - fewer strings is way easier, in one sense at least. Because every time you ring one string you need to mute all the others. Fewer strings to mute means easier, in isolation.
Another argument that the less musically experienced might rely on, but not such a strong one in my opinion - most songs you hear have simple bass lines. On the surface, that seems like it translates to easier, at least, if not actually easy. But that's a misunderstanding. Many bass lines are simple not because the instrument itself is easy, but because of ways that it's actually more demanding. It's easier to get away with something melodically complicated but loosely timed on virtually anything besides the Bass. It's much harder to express yourself while still holding down the rhythm section function, as opposed to doing so in free time over a solid rhythm section.
So while the Bass *can* expand beyond the rhythm section functions in the hands of an expert, it's much easier to do that with virtually any other instrument. Very much not easier.
Physicality comes into it too. Six blades are very finely tuned to the size of the folks that tend to buy guitars. And with six strings to worry about, often expecting several to ring at once, this is important. Relatively easy if your hands fit, relatively hard if your hands are larger.
Bass strings aren't really spread much wider. BUT you virtually never need to fret one while another rings further up the board, as is so common on six string. And the frets are more widely spaced, of course. Whether this makes one or another easier or harder though? Entirely dependent on your own dimensions and what you want to do. I can fret cleanly all the way up every Bass I've tried it on, I often can't do that with a six string, especially a 24 fret, at the 24th fret the distance between frets is much less than my finger takes up, it's not impossible to hit the note but it's virtually impossible to do so quickly and reliably. If I center my finger where it's supposed to go, I mute the note. If I press it down just right, behind the 23rd fret and rolling up, that works, but it's slow and awkward.
That's the main point, I think, it's not easier or harder, without context you can't tell. Context is always key.
But the consequences of giving generations of kids a hasty generalization as if it were absolute truth? Well that's another layer to this. There have been some incredible bassists, even as the trajectory has been for it to be where you find the least talented and least capable member of each band.
So I'll leave you with a positive example. Perhaps a slightly older one than you were expecting.
Perhaps one more topical in 2019 than you imagine as well.
https://www.youtube.com/watch?v=5CUeI9funsk [The Who - Won't Get Fooled Again - Studio/Recorded]