"We asked" is the simple and straightforward answer that Lars and Joe give to guests, as well as the hosts of other podcasts they go on, when asked how they have gotten such huge guests on The First Podcast.
The First Podcast has had some proportionally absurdly large guests on, from Pogo on the very first episode, to the literal creator of the Elder Scrolls, to CallMeCarson, to California's former State Senator Josh Newman, to the creators of Cyanide & Happiness. Guests of this size and importance would be right at home on a much larger podcast.
The reason Lars and Joe were able to have them on is because they asked. It really is that simple.
Several people have noted that this answer feels inadequate, as it seems like "there should be more to it". In reality, however, the boys found an email address, sent an invitation, and got a yes.
Lars hypothesizes that, while assuredly the big guests they've had on get asked to appear on things on a regular basis, there are probably a lot of people who don't even bother, since they assume everyone else is already asking.
What this means is, hypothetically, the inboxes of the guests are not as full as one might think, and thus it is not unrealistic to assume some may see said email.