Patents certainly do get checked, but benefit of any doubt as to their validity, prior to grant, is given to the applicant. This is because Patent Examiners do not have infinite resources or infinite time to conduct exhaustive searches or do experiments to determine the absolute novelty, non-obviousness and practicality of claimed inventions . In other words there is, at the grant stage, a presumption of validity.
The examination process is, in effect, a first stage sieve. It is then down to the Courts to rule on validity if the patent is challenged.
For an entertaining light-hearted read (not!) before bed-time, check out the examiners'
Manual of Patent Practice.