yes it is true, only Shadow of the Colossus, MGS3 and Hitman made decent use of the console, all those 60fps games you know could have been even better at 25~30fps all them.
There are many factors, whether related to planning or causality.
The initial dev kits were broken, generating games that were noticeably below what we see in future games with the exception of Tekken Tag where the developer really defended the honor of the series his merit, not the dev kit's.
the causal reasons were strictly commercial, it was enough to make a functional game and sell it
an example was Sonic Heroes, a game completely made out of spite, it sold almost 3M more on the ps2 than the sum of the other versions and the examples only grow.
this was the central reason why the ps2 was almost unexplored.
Difficulty programming
Working with the PlayStation 2 required time and money, its texturing system required care that most devs were not willing to take, making proper use of VU's was not something that many devs did. Despite, the developer called ERP said that there was a positive point, updated dev kits gave the developer a fabulous range of activities, almost everything could be done, so we return to the initial question again , reasons strictly commercial.
Conspiracy theory
There is a theory that Sony vetoed the use of techniques that imitated shaders and bump mapping in order to present such techniques as new on the PS3.
Yes, the PS2 didn't have dot3 but due to the fill rate the PS2 could implement this technique, completely modifying any game but Sony vetoed.
Another supposedly vetoed technique concerns texturing. The technique was developed in 2005 and would be a game changer, but at the end of 2006 Sony would launch the PS3.
PS2 (contrary to popular belief) was the console that had the least correctly used hardware in history.