Don't. Don't try to force your students to attend. Your students are adults and they can make their own decisions. They are voting with their feet, signalling that they don't see attending the seminar as the best possible use of their time. Maybe they are right, maybe they are wrong, but that is their decision to make. I think you should support them and applaud them for taking care of themselves and prioritizing where to spend their energy. They might be making a mistake, but we all need to make our own mistakes and learn from them.
It's worth remembering your relationship to them: you are their advisor. It is their PhD. They will get out of their PhD program what they put into it. They are in charge of their progress, and are responsible for the results. As an advisor, you can give them advice. But it's not your place to force them to attend seminars. It's ultimately their decision about whether to follow your advice, or not to.
I suggest you think about how best to serve them. What will best help them in their career? How can you best serve their needs? That might be creating opportunities, offering teaching and feedback, giving helpful advice, helping them make connections, etc. If you start from the premise that your goal is to get them to attend the seminars, that risks leading you down an unhealthy path; whereas if you start from the goal of serving them, I think it sets you up for a better perspective.
You believe that they will be better off if they attend the seminar. I suggest that you spend a little time reflecting on how and why it will benefit them. Also, make sure to take into account the opportunity cost: there are other things they could be doing with that time, and why is this more valuable than the other valuable things they could be doing? For instance, if they want to go into academia, maybe hearing a few job talks from faculty candidates in other areas is helpful, because it will help them prepare to give their own job talk, and help them empathize with an audience who is not an expert in the subject of the talk? Maybe there is a particular speaker who is doing work that is essential for everyone in your area to be familiar with? If you think you can make a solid case that it's better for them to attend the seminar, then talk to them, share with them your advice and your reasons (once), and let them make their choice accordingly, and respect their choice. Don't try to penalize them if they have a different perspective.
I also suggest that you think more about attraction and less about coercion. In other words, if you think it'd be good to have more students attending a seminar, think about how you can make it more valuable and attractive to attend. Can you make it more inspiring or more useful in some way? Perhaps you might consider having the students organize and lead the seminar, so they can choose topics and format that will make sense to them? Maybe it'd be more valuable if your students were the speakers, speaking about their own work or some paper they enjoyed reading, rather than inviting in outsiders? Maybe not all speakers who are attending are exciting to everyone to everyone in the group -- if so, that would be normal. In many universities, there are lots of opportunities to attend talks; maybe your students feel they have enough of that and don't need more. Is there something that they would value and they do need? Or perhaps a difference pace (e.g., every other week rather than every week) would fit people better? Finally, there is a chance it might help if you participate, you provide high-quality food, and you "set a tone" in positive atmosphere and supportive culture -- you may already be doing all of that.