This is a public policy question, which you may find answers to in legal/economic literature.
Just wondering why there is no requirement for foreign entities
seeking to patent an idea / technology in a territory be obliged to
manufacture, license and / or offer to sell a product based on the
patent in the territory?
This is largely true, but there may be some exceptions. See for instance, compulsory license.
Patents obviously grant a monopoly on a particular technology, for a
defined period. Primarily to allow the inventor time to recoup the
development costs and profit from their labours, and to foster further
innovation, but if an entity simply seeks a patent to block / prevent
a technology being used in a territory for 20 years, what possible
benefit is there for the territory to grant the monopoly?
The general 'trade' is that an inventor discloses useful information to the public (a 'novel', 'non-obvious', and 'enabled' disclosure such that one having ordinary skill in the art can practice), and the government grants temporary offensive rights to go after others. Therefore, the primary intended benefit is that one inventor can inform others such that they don't have to re-invent the wheel. However, in practice it does not seem that patents are the 'go-to' source of information one would consult before inventing, in part because patents aren't optimized to 'teach', but rather are optimized to secure as broad of rights as possible with minimal effort. Additionally, knowledge of prior art may have deleterious effects in pursuing rights to one's own invention (e.g. duty of disclosure, inequitable conduct) or practicing in that area (e.g. treble damages).
If there is no conceivable benefit then why isn't there an explicit
requirement that an entity seeking a patent must either: manufacture,
license and / or offer to sell a product based on the technology in
the territory, within a defined time frame, or the patent will expire.
First of all, patents aren't exactly "free" in terms of fees (USPTO and/or laywer) and time. They cost at least $5K on the low end, and possibly up to $20K (esp. if you include Office Action responses). However, even assuming patent costs weren't a concern to filers, here are some reasons we might not want to require practice of claimed inventions:
- doing so would couple the action of 'inventing' to business execution (which would disallow one to specialize as an 'inventor'), or at minimum may require some inventors to license their patents for less than desired, foreclosing on the option to wait until a better buyer came along
- doing so may prohibit the 'little guy' from getting into the inventing/business game at all if competitors can box him/her out of practicing by other patents and/or economies of scale; with the current system, the little guy can perhaps strike a cross-licensing deal to gain access to the business or license the patent; there definitely seem to be abuses to this though, i.e. trolls
- what if someone intended to take their claimed invention to market, but were only willing to do so if they were guaranteed patent protection?
- would you require operation immediately at application or upon grant?
- what would be the metric for 'being in business'? what if it took them years until they were able to commercialize the claimed invention? such a determination would get messy
- if you force a non-practicing entity to license/sell their patents, how would you avoid them from making ineffectual such a requirement, e.g. by making the license/sale fee $1B+? would you have courts determine a "reasonable" license or price? this would similarly get messy
While I personally would prefer that patent rights were weakened (I operate in the electrical/computer area which requires less patent incentive to invent as opposed to, e.g., pharma), it would seem difficult in practice to enforce practice of claimed inventions as you suggest.