Sperm can survive in the female reproductive tract for several days and pregnancy can occur when a single episode of intercourse occurs up to five days before ovulation. However, when intercourse occurs one day or more after ovulation, pregnancy is extremely unlikely that cycle. Therefore, it is better to err on the side of having intercourse before ovulation rather than after. Once a positive urine LH surge has been detected there is no benefit to repeated testing in subsequent cycles to time intercourse. This can become expensive and as long as the menstrual cycle length does not significantly change, ovulation can be presumed to be occurring in subsequent cycles.
Human reproduction is inefficient.
The chance of conception can be significantly lowered by other factors such as advanced age of the female, endometriosis, low sperm count, etc.