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. Here 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
