I've watched clips and it seems the referees in sambo allow sambo practitioners a better chance and more leeway for grappling.

Sometimes it seems like Judo refs stop it prematurely.. What's with that? And what's with guys that standup voluntarily after hitting the mat even though they did the throwing? *scratches head*

It can be advantageous to have it broken up quickly. At the end of the day, as long as you are trying to get out of it, the ref will let it go. It is good Ne Waza to get out of seemingly tricky situations as fast as possible. Frankly, if you are allowed a long time to get out of a ground hold, that can be a disadvantage. You will get use to taking your time. A sport like Judo makes you act quickly, and if you know you are against the clock, you will practice getting out quicker.

Don't underestimate the impact of the Olympics on Judo too. If you have a 5 minute round, and a potential audience of billions, you can't have 2 players staring at each other on the ground for 45 seconds. People will change channels.

Can I ask mudo, do you find that quick breaks in Ne Waza happen to you, or is this a general observation of Judo?Reason I ask that as a Judo newbie, your Ne Waza will take a little time to get up to power, and so you will find yourself in places you can't get out of at the start of your Judo journey. We all went through it. Most of my early randori was broken up, more times than I care to remember. The longer you practice, the longer uninterrupted randori you will have.
"Let your food be your medicine, and your medicine be your food" Hippocrates.