Australia, New Zealand and Canada have a rather straightforward ‘points system' under which a person who has a set of qualifications which add up to a given standard are automatically granted work permits and/or immigrant status.
Currently and with a firm job offer or contract I qualify for any of those countries, since I fulfill with good points the other requirements.

The United States has a much more complex and time-wasting system under which the employer must justify that a given foreign worker is needed.
Often the issue is that such a foreign worker must have some extraordinary skills.
As a Uruguayan national I do not need a visa to enter the United States, nevertheless to enter full-time employment I do need a worker's visa (the infamous ‘green card'). If I am offered employment in the US we will have to seek it.

H1 H-1B	Sec. 101(a)(15)(H)(i)(b) of the I&N Act as added by PL 101-238, Sec. 3(a) (Dec. 18, 1989); revised by PL 101-649, Sec.205(c) (Nov. 29, 1990)	Temporary worker (other than registered nurse) with "specialty occupation" admitted on the basis of professional education, skills, and/or equivalent experience.
H2 H-2B	Sec. 101(a)(15)(H)(ii)(b) of the I&N Act as added by PL 99-603, Sec. 301(a)(b) (Nov. 6, 1986); revised by PL 101-649, Sec. 205 (Nov. 29, 1990)	Temporary worker performing services or labor unavailable in the United States (including Spanish sheepherder, excluding agricultural worker).