Two reasons:
1) Most Malaysian Chinese live in developed urban cities where the cost of living is much higher compared to the rural kampungs. A typical security guard pay in Malaysia is around RM1,500 and total package of RM2,000 if you include the other perks and allowances, that is not enough to maintain a decent standard of living in the bigger Malaysian cities.
2) Most of their GLCs, big local companies and foreign MNCs try to ensure they hire at least 30% - 50% of their staff as Malays to demonstrate their "corporate social responsibility" to the government. For the PMET and senior management jobs, there are very little qualified Malays, so companies meet their quota mostly in simple unskilled jobs like security guards. Most Malaysian companies do not want to waste their "brain dead cannot cause damage" job slots for the Chinese.
I used to work in a big foreign MNC, their Malaysian subsidiary hire 3000+ people of which the proportion of Malays in PMET levels is only ~10% and there is a grand total of 1 Malay who reach the senior management level. When it comes to jobs like warehouse assistants, drivers, security guards, basic admins and junior production operators, it's almost entirely Malay with the occasional Indian. The fact that we are on paper hiring 70% of the people as Malays gives us leverage when negotiating work passes, dealing with the unions and tax department.