It's not that strange for people to be elected governor without previous political experience. The big reason why Ventura and Schwarzenegger are different is that they were famous in the entertainment industries. This was also true of Ronald Reagan when he was elected governor. I don't know of a fourth example, but I also don't know that there isn't one. It's unclear if the entertainment industry would have supported such a race prior to the twentieth century. Entertainers who are household names are relatively new. Prior to movies most entertainers played to live crowds which were rather small.
Most non-political governors are business people. For example, Rick Scott, Rick Snyder, and Matt Bevin are examples of governors whose previous experience was as business CEOs. Tom Wolf had previously held an appointed position before being elected. According to this article from the Washington Post, only two governors in 2014 had never held elected office previously: Scott and Snyder (Bevin was elected after that).
There are also many career politicians who become governors. For example, John Kasich, Bobby Jindal, Jay Inslee, and Mike Pence were members of the federal House of Representatives. Sam Brownback was in the federal Senate. Scott Walker was a county executive. Nikki Haley was in the state House. Maggie Hassan was in the state Senate. Those are all current governors.
It's rarer for business people to make the direct jump to President. The only one was Herbert Hoover, and he held a cabinet position (appointed not elected) prior to running for President.
There's been a long history of generals becoming President without holding other political office. For example, George Washington, Zachary Taylor, Ulysses S. Grant, and Dwight Eisenhower. Martin Van Buren and John Quincy Adams never held elected office before the Vice-Presidency and the Presidency respectively (Adams did hold elected office afterwards and was an appointed Senator previously). But both had quite a bit of appointed experience. Andrew Jackson and William Henry Harrison both held political offices as well as military ones.
The most common road to the presidency has been through a governorship. George W. Bush, Bill Clinton, Ronald Reagan, and Jimmy Carter had all been governors. Vice-Presidents are also common. George H. W. Bush, Richard Nixon, Lyndon B. Johnson, and Harry S. Truman were all Vice-Presidents before being elected President. In that same period, there have also been two Senators who became President: Barack Obama and John F. Kennedy. Truman, Johnson, and Nixon were also Senators prior to being Vice-President. George H. W. Bush had been in the House.
I don't know if it's becoming more or less common for outsiders to become governor (there's only the one example of an outsider President and that's from 1929-32). Most governors have a political background of some sort. So it's not a lot more common. It's also not clear to me that it used to be uncommon.
Donald Trump has never held political office and would be the first President to be elected without political or military office. Of course, it's not clear that he'll win the Republican nomination much less the presidency.