The Boeing 737 NG series first flew in 1997, but the fuel inerting system was only certified in 2006 according to b737.org.uk (emphasis mine):
Centre Fuel Tank Inerting
To date, two 737's, 737-400 HS-TDC of Thai Airways on 3 Mar 2001 and 737-300 EI-BZG operated by Philippine Airlines on 5 Nov 1990 have been destroyed on the ground due to explosions in the empty centre fuel tank. The common factor in both accidents was that the centre tank fuel pumps were running in high ambient temperatures with empty or almost empty centre fuel tanks.
Even an empty tank has some unusable fuel which in hot conditions will evaporate and create an explosive mixture with the oxygen in the air. These incidents, and 15 more on other types since 1959, caused the FAA to issue SFAR88 in June 2001 which mandates improvements to the design and maintenance of fuel tanks to reduce the chances of such explosions in the future. These improvements include the redesign of fuel pumps, FQIS, any wiring in tanks, proximity to hot air-conditioning or pneumatic systems, etc.
737s delivered since May 2004 have had centre tank fuel pumps which automatically shut off when they detect a low output pressure and there have been many other improvements to wiring and FQIS. But the biggest improvement will be centre fuel tank inerting. This is universally considered to be the safest way forward, but is very expensive and possibly impractical. The NTSB recommended many years ago to the FAA that a fuel tank inerting system be made mandatory, but the FAA have repeatedly rejected it on cost grounds.
Boeing has developed a Nitrogen Generating System (NGS) which decreases the flammability exposure of the center wing tank to a level equivalent to or less than the main wing tanks. The NGS is an onboard inert gas system that uses an air separation module (ASM) to separate oxygen and nitrogen from the air. After the two components of the air are separated, the nitrogenenriched air (NEA) is supplied to the center wing tank and the oxygenenriched air (OEA) is vented overboard. NEA is produced in sufficient quantities, during most conditions, to decrease the oxygen content to a level where the air volume (ullage) will not support combustion. The FAA Technical Center has determined that an oxygen level of 12% is sufficient to prevent ignition, this is achievable with one module on the 737 but will require up to six on the 747.
The Honeywell NGS was certified by the FAA on 21 Feb 2006 after over 1000hrs flight testing on two 737-NGs. Aircraft have been NGS provisioned since l/n 1935 and it has been installed since l/n 2620 onwards. The NGS requires no flight or ground crew action for normal system operation and is not dispatch critical.
So all older NGs were built without an inerting system. According to this article, the FAA required new models since 2008 to have an inerting system and retrofitting of all 737s until 2018 (emphasis mine):
The headaches are hitting Boeing 737 operators, which make up the bulk of the early adopters among those required to comply with the FAA’s Fuel Tank Flammability Reduction (FTFR) rule. The rule, passed in 2008, called for new-production versions of the affected models—737s, 747s, 757s, 767s, 777s, Airbus A300s/A310s, A320-family models, A330s and A340s—to have fuel tank inerting or flammability reduction systems installed starting in late December 2010. It also required airlines to retrofit half of their affected fleets with an approved system by 2015 and complete the rest by 2018. The phased compliance led operators to prioritize their largest subfleets to ensure they met the first deadline, putting 737s at the top of the list.
It is quite possible that not every airline worldwide has completed this retrofitting.