First, the old strips (80s to mid 90s) were much funnier. Often they were downright absurd or even surreal, and that type of humour was not common at the time, especially when it mixed up or combined all those aspects.
Second, back in that time, you couldn't check one bajillion webcomics on your phone. The strips appeared either daily in your morning newspaper, or collected biweekly / monthly in comic book releases in some countries. You had to pay for them. And as far as paid humour comics went, Garfield was among the better ones.
The humour itself was also universal in that it appealed to all ages. They weren't kid-only humour, or adult-only humour. Kids liked it for how wacky it was, adults could like how much of a cynical dick Garfield was or how much of a loser Jon was.
Point is, you didn't need to understand any origin stories or know each characters quirks or anything. Jon is a loser single guy who owns two pets.
And consider that this is a DAILY strip. It doesn't have to hit a home run every day. It's purpose is to bring a smile to your face once a day. It does that fine.
The characters were also extremely marketable, and Jim Davis took advantage of that.