I'm pretty sure the answer is no, but is there any way around the 25k limit on individual CSV import files?

There is ni such limitations in netsuite , if we upload more you can get more errors

Why not use a tool like Celigo in that case?

Long story @KevinJ of Kansas, but we do have a tool we could use to shove transactions in. And that is the plan longer term. That being said, we're under a time crunch to make changes to this process and it would be way easier short term if there was some way to increase the line limit per file.

Nothing that wouldn't take more time than switching to a new tool I imagine.

You can use the free dataloader that comes with a free Celigo account

is anyone aware why we have such limit? Its very small in today's understanding. Could it be that its just relict from ancient ages when Netsuite was born?

I'm guessing like you said it's a historical thing and it's probably not enough of an issue for enough people that they're willing to dedicate dev time to it.