I worked as a software engineer for 20 years in a computer science department and there are several reasons it is often necessary to work on weekends:

* Paper deadlines. If everyone planned well there would be no need to work on weekends, but nobody plans well, so a lot of the work is done last minute. Also research is international and as a result many conference paper deadlines are set randomly so as not to always be convenient for USA and inconvenient for everyone else. Hence some deadlines are on weekends. Again, if the work was well planned there would be no need even then to work on a weekend, you could submit the paper before the weekend, but no one plans well.

* Students time schedule. Students have classes. They sleep late and work nights. They go to parties and drink. They have exams and assignments that take all their attention during the week and then they get back to the research on weekends. If you are collaborating you often have to be connected in real time, even if it is remote work. This means working nights, weekends, really any time of the day. It can be like being on call.

* Profs and advisors try to get you to cram as much as possible into the paper to make it better. This means working up to the absolute last minute continuing to make minor revisions. I've seen changes made to papers in the last seconds before a deadline, and changes continue to be made for the hours grace period some conferences offer after the deadline. As long as the paper submission portal is open changes continue to be made. It's a stupid way to work, but it is common.

That said, since I was an employee I had more freedom to restrict my hours. Grad students on the other hand are slaves and are expected to work 80+ hour weeks. They are essentially almost always working. So they will try to access other people any time they encounter an issue. It's difficult to not interface with them during "off" hours because you know it might mean they can't get anything done until they talk with you, which can mean wasting days of time for them. They'll mention this at meetings, which makes it look like YOU are blocking the project. And you are, but for reasonable reasons. But that doesn't look reasonable to the student. And profs used to be students and in many cases that's basically the only life they know so they tend to sympathize with the students and not with you.

Some things that would help:

Plan for papers, posters, meetings so it's not all last minute. The conferences happen around the same time every year. It's not like it's a big surprise when the date is set.

Get grad students to form unions so they are not so overworked.

Set rules on what profs can task grad students with. Getting coffee, creating lessons for the profs classes, getting the profs laundry, being volunteered for work, being assigned more than 40hrs work/week, should not be on the list. This is a large part of why grad students are overworked.

Require management courses for profs so they learn that people are not machines. There is a reason we have a 40hr work week, and while going beyond 40hrs is _sometimes_ required, working longer hours continuously just damages productivity and is actually making the work go slower and burning people out.