Doubt in understanding the time complexities of algorithms to recognize regular expressions
The authors are describing the time complexity to determine whether the input string $x$ is in $L(r)$ in the table and not including construction time, that is they list the per-string cost of using the DFA or NFA, and not the one-time construction cost to build it. In many cases the construction will be done once, and used to recognize strings many times, so it may be worthwhile to sacrifice some time upfront to gain more efficient string recognition afterwards.
You are right to consider the construction cost though. If the automaton will be constructed and used only once, or a small number of times, then the savings in string recognition efficiency may not justify the higher construction time costs.