As @Jeff mentions you are essentially asking why people stereotype. Steretyping is a huge topic particularly in social psychology. The wikipedia article on stereotypes provides a basic introduction to the functions of stereotypes where it talks about cognitive and social functions. It seems like most of the examples you provide are referring more to the cognitive functions (e.g., simplifying thinking and decision making).
An article by Macrae et al (1994) typifies this perspective. You might want to have a read of the article. HeresHere is an extract from the opening paragraphs that talks about stereotypes as tools for minimising the use of cognitive resources.
Social psychologists have frequently characterized stereotypes as energy-saving devices that serve the important cognitive function of simplifying information processing and response generation (e.g., Allport, 1954; Andersen, Klatzky, & Murray, 1990; Bodenhausen & Lichtenstein, 1987; Brewer, 1988; Fiske & Neuberg, 1990; Tajfel, 1969). Building on this tradition, Gilbert and Hixon (1991) aptly characterized stereotypes as "tools that jump out" of a metaphorical cognitive toolbox "when there is a job to be done" (p. 510). Anyone who has ever succumbed to the temptation to evaluate others in terms of their social group membership would doubtlessly recognize the power of this contention. Individuation, in its many guises, is a rather time consuming and effortful affair (Brewer, 1988; Fiske & Neuberg, 1990; Fiske & Pavelchak, 1986). Stereotyping, in contrast, relies only on the execution of some rather rudimentary skills: most notably, the ability to assign people to meaningful social categories (see Hamilton, 1979; Hamilton & Sherman, in press; Hamilton, Sherman, & Ruvolo, 1990; Hamilton & Trolier, 1986). Once achieved, this categorization provides perceivers with a veritable wealth of stereotypic information.
The metaphorical view of humans as cognitive misers has attained a zenith of popularity among contemporary social cognition researchers (see Fiske & Taylor, 1991; Higgins & Bargh, 1987; Sherman, Judd, & Park, 1989), but the notion of stereotypes as simplifying mental devices has its origins in much earlier times. Lippman (1922), for example, argued that reality is too complex for any person to represent accurately. Stereotypes, accordingly, serve to simplify perception, judgment, and action. As energy-saving devices, they spare perceivers the ordeal of responding to an almost incomprehensibly complex social world. Seventy years later, these sentiments are characteristic features of cognitive writings on the topic. As Fiske and Neuberg (1990, p. 14) remarked, "we are exposed to so much information that we must in some manner simplify our social environment. . . for reasons of cognitive economy, we categorize others as members of particular groups—groups about which we often have a great deal of generalized, or stereotypic, knowledge.
I also think Tyler and Ana make good points about what is meant by statements such as "Russians like vodka". One interpretation is that the exceptions are implied. Another is that when we say "Russians" or any other group we are referring to the group as an entity. Thus, to say that a group has a certain property is to speak about the tendency of the individuals in the group rather than each individual.
References
- Macrae, C. N., Milne, A. B., & Bodenhausen, G. V. (1994). Stereotypes as energy-saving devices: A peek inside the cognitive toolbox. Journal of Personality and Social Psychology, 66(1), 37. PDF