In most (or at least many) fields of academia, peer-reviewed publications are essential. For a compilation thesis, no papers means no PhD. For tenure, you need papers. To get grants, you need papers. Universities may distribute internal funds based on the number of papers per group. In short: publish or perish.
On the other hand, it is quite cheap to offer someone co-authorship. Send a nearly finished manuscript to a colleague/friend at another university for review... colleague reads it, offers some advice, perhaps just minor. First author offers co-authorship in return, and colleague has another co-authored paper for possibly less than a day of work. One can discuss if it is the right thing to do, but that is not my question here. It happens. (NB: I am not suggesting such has happened in the examples listed below!)
Criteria for co-authorship differ per field, but some papers have a lot of co-authors. Perhaps due to having an instrument that was used in an inter-comparison/validation study. Some examples of papers with lots of co-authors, not particularly extreme:
Kasai, Y., Sagawa, H., Kreyling, D., Dupuy, E., Baron, P., Mendrok, J., Suzuki, K., Sato, T. O., Nishibori, T., Mizobuchi, S., Kikuchi, K., Manabe, T., Ozeki, H., Sugita, T., Fujiwara, M., Irimajiri, Y., Walker, K. A., Bernath, P. F., Boone, C., Stiller, G., von Clarmann, T., Orphal, J., Urban, J., Murtagh, D., Llewellyn, E. J., Degenstein, D., Bourassa, A. E., Lloyd, N. D., Froidevaux, L., Birk, M., Wagner, G., Schreier, F., Xu, J., Vogt, P., Trautmann, T., and Yasui, M.: Validation of stratospheric and mesospheric ozone observed by SMILES from International Space Station, Atmos. Meas. Tech., 6, 2311-2338, doi:10.5194/amt-6-2311-2013, 2013.
Milz, M., Clarmann, T. v., Bernath, P., Boone, C., Buehler, S. A., Chauhan, S., Deuber, B., Feist, D. G., Funke, B., Glatthor, N., Grabowski, U., Griesfeller, A., Haefele, A., Höpfner, M., Kämpfer, N., Kellmann, S., Linden, A., Müller, S., Nakajima, H., Oelhaf, H., Remsberg, E., Rohs, S., Russell III, J. M., Schiller, C., Stiller, G. P., Sugita, T., Tanaka, T., Vömel, H., Walker, K., Wetzel, G., Yokota, T., Yushkov, V., and Zhang, G.: Validation of water vapour profiles (version 13) retrieved by the IMK/IAA scientific retrieval processor based on full resolution spectra measured by MIPAS on board Envisat, Atmos. Meas. Tech., 2, 379-399, doi:10.5194/amt-2-379-2009, 2009.
(Again, I would like to stress than I am absolutely not implying that there is anything inappropriate about these two examples!)
On the other hand, I rarely see papers written by sole authors, and I have the impression that such papers were more common in the past — but I have no evidence thereof.
Is there an inflation in the number of authors per paper? In other words, is the number of authors per paper increasing and if so, does this reduce the value of a co-authored publication?