You'd have to define "need", natch. Thermal management is usually part of the design process and includes data on component lifetime and overall system lifetime and failure rate (MTBF - mean time before failure) as well as cost and performance trades. Without saying they are right or wrong, valid, worthwhile or not, here are some reasons off-the-cuff:
- Thermal stability can also be important, as in keeping the temperature from changing (much) after reaching normal operating state. Precision clock sources usually include an oven (heater) to provide a stable temperature-controlled environment. Device (transistor, resistor, etc.) performance varies with temperature so sometimes it is better to let it warm up and stay there instead of having random airflow change the temperature. Yes, well-designed components should not, but OTOH the cost goes up when designing for that sort of stability (which is usually much better than needed for audio, at least IME).
- Related to above, sometimes you want to control the thermal environment, so ventilation holes and airflow are controlled to direct the air where you want it and not to other places that might cause thermal shifts and such. That precludes just drilling holes everywhere, or randomly enlarging them, etc. If cooling was all that mattered, you could just take the top off and place a little fan blowing on the board(s). But consider that a lot of electronic things, like your PC and enterprise servers, are designed for proper airflow with the covers in place. Remove them, and the server will overheat (I am around such things in my day job and have seen it more than once -- removing the cover to work on them causes them to overheat as the air from the fans leaves the case instead of being forced through the air tunnels to cool the chips inside).
- There is also shielding to consider, one reason for not taking the top off, and more and larger holes can lead to EMI/EMC issues (ingress and egress of undesired signals). That is one reason a lot of little holes are used instead of fewer, larger holes -- less LF leakage (in and out).
- Holes and slots cost money in manufacturing the cases and restrict component placement inside the case. Adding more might not improve things (see points above) and just cost more money. Components may be place such that additional holes do not help, e.g. on the bottom when there is a circuit board above that blocks airflow from that direction anyway.
- Drawing cold air in also means drawing in dust and such that can coat the components inside, leading to worse thermal issues than if the airflow was more restricted, and affecting circuit performance both through heat and potentially leakage paths (a worst-case example would be dust and grunge leading to arcing on a high-voltage board and failure of the device).
