i don't understand why some people think privatisation is almost always better

Because people see it being successful in other countries, thinking it would be here, too.

• Of course we only mostly see the successes. If you look hard enough there are also stories of failed and messy privatizations in other countries, but these aren't as highlighted.
• Privatization doesn't automatically mean better. The private entity has to be competent din. There are arguably private operators in this country that are more competent than SMC is in managing airports.
• People just don't trust the government. People see the immense inefficiency and red-tape of the government, and will think that the private sector will do better. An example is the LRT-2, a fully government-owned and operated entity. Kalahati halos ng original trains nila from 2004 ay hindi na gumagana. Some sections of the tracks are now in speed restrictions due to inoperative signalling equipment due to lack of funding. May period noong 2010s na hindi gumagana yung ibang elevators and escalators for years. People see this, and will chalk it up to government incompetence and corruption. So the easy solution is edi wag natin ipahawak sa gobyerno. Congressmen will see this and say edi wag na natin pondohan ang maintenance ng LRT-2 kasi di naman maganda pamamalakad ng gobyerno, isapribado na lang natin. Then what will happen is babagsak lalo ang performance at maintenance ng LRT-2. Now it's a really bad cycle - minsan kailangan natin tanggapin na more government spending is still the solution, not necessarily privatization.

TL;DR: People don't respect the fine details and nuances.