Some conferences are so highly selective that a small fraction of conference-goers give presentations, but other conferences are less selective and the majority of attendees give presentations of some sort (talk or poster). These less selective conferences are excellent opportunities for students to present their research, but many senior researchers also give presentation.
If you have ever attended the type of conference at which most participants give a talk or poster, have you ever decided to just attend without giving a presentation, even though you easily could have submitted an abstract for review (with you as presenting author)? If so, why did you decide to be a Conference Tourist (CT)? Was it one of these reasons, or something else?:
A1. Fatigue. You have given so many presentations at so many conferences, you just wanted to go to the conference without having to prepare anything to present. Skipping one (or tw0?) conferences doesn't mean you are totally burned out; you just want a break and will enjoy the conference as a CT.
A2. Stress. This is similar to Fatigue, but in this case you decided against trying to give a presentation because doing so always stresses you out so much that you spend all the days and nights before your presentation feeling anxious, and you can only begin to enjoy the conference once your presentation is done.
A3. Lack of anything new to say (not that that reason stops some people from giving presentations). Somehow, a year has gone by and you don't have any new results. You will soon, of course, but you didn't have anything to write up in time for the abstract deadline and you didn't want to present old or recycled research. This is not the best of reasons to be a CT, but it might be fine to do once in a while. Not everyone's research projects fit exactly with conference submission schedules.
A4. Been there/done that.You are happy to let your students and postdocs present all the results from your group. Let the youngsters have all the glory (and stress). You can sit back and be the big cheese research group leader.
A5. None of the above.
And then, just to turn the question around: If you could easily go to a conference as a non-presenter but you never (or almost never do), why do you so often submit an abstract or conference paper for review?:
B1. You love giving talks. You are addicted to the thrill of presenting your research to a large audience. It would be painful for you to attend a conference and not give a presentation and be part of the action.
B2. You like giving talks. That is, you don't love giving talks, but you like it well enough that, if you have some interesting new research to present, you want to present it at the conference.
B3. You may or may not like or mind giving talks, but you feel compelled to give a presentation if at all possible because you want to show funding agency program officers and others that you are being productive.
B4. You have some great new research results and you want the world to know this now, not n months from now when (you hope) the paper is published.
B9+ Some of the above/none of the above..?
My answer for the latter set of questions would be one involving parts of B2-B4.