Political parties tend to avoid, whenever possible, turning "Safe seats" into potential "Swing seats"
There's a few issues which help explain why there's the primary challenges mentioned above - and the name recognition, that boil down to why the party themselves try to avoid making the strategy of moving to safe seats to run for a candidacy hard in the first place.
Changing out your incumbent for a newer candidate is always a riskier move than continuing with your existing incumbent; you've built up momentum over elections to solidify their "safeness", and this allows you to know how people in the riding react to the incumbent. That is, they vote for the incumbent.
Now granted, that's not always a given - sometimes an incumbent resigns, or tries to aim for a Presidential run, and you need to account for a replacement, or maybe you're looking for a replacement because an incumbent died. But until then, a political party can spend less on the incumbent's campaign and get desirable results given the benefits of spending in the previous election, so there's not much incentive to change horses in mid-stream. It's even a part of the message a political party can leverage - if you like what the incumbent candidate does for the riding, vote for them again.
As a result, fundraising from a "Safe seat" can be allocated to existing "Swing seats", and help in more difficult ridings. The incumbent has a proven track record of getting some funding from their supporters, and can use the existing groundwork they've done to help out with this. Or in short - people give fundraising donations to candidates they already know and trust.
Swapping out an incumbent candidate for an up and coming candidate means that you have to start from scratch on all of that - doubly so if the up and coming candidate is from out of the riding. Campaigning from scratch is basically about getting your name out there, and if you're from the riding, more people will already know your name. If you give up that advantage, your opposition party runner basically becomes the more well known name in the election, turning your seat from "Safe" to "Swing", because even if you have the same policy positions as the previous incumbent, everybody already knows who's trying to run against you.
Even in a solidly safe riding, you don't want to have to get caught flat-footed and find out that a majority of the voters who appeared to give you a "Safe" seat were in fact swing voters who just voted for your incumbent.
Party unity is easier to show when you aren't replacing an incumbent in a "Safe seat", because you're supporting a candidate that already is voted into that riding - shaking it up when you lose a riding is a sign that you're attempting to adapt and address concerns people have in the riding.
There are times when you'd want to pivot to a new issue in your riding - and it's usually something you'd want the incumbent candidate to run in so that they can pivot to it within their existing platform.
If you're the challenging party, this is a bit easier to swap out the candidate - if they lost, what you want is to find a candidate that can support your party in the riding - in a way to make the riding go from "Safe for incumbent" to "Swing for our party instead".
If you're challenging an incumbent within your own party, you need a strong reason to have an incentive to change the running platform - which is hard either way, but easier if you're from the riding itself, and can say "These are policies I think we should implement to tackle problems our incumbent position is not already."...but either way, you're running the risk that you're making the party seem like it's infighting over policy minutia. Political parties may be fighting over policy minutia internally, but you don't want to give the outward opinion of it. For that reason, primaries and caucuses are there to allow the party to come to a unified opinion, and to try and reduce the possibility of people within the party disagreeing on election day - and just writing in their preferred candidate that didn't win those discussions, and split the vote.