It's not that the refs just 'ignore it' now - they changed the rules around it. Taken from Wikipedia, when discussing how a scrum was originally setup to have the two hookers competing for the ball:

Around 1996, the All Blacks came up with a strategy where their pack would push at the same time the hooker would strike for the ball. This not only meant they got the hooked ball, but also the opposition had to retreat to stay onside.[22]

Eventually, other teams found out what the All Blacks were doing and copied them. Referees stopped enforcing the straight feed into the scrum, and as a result scrum-halves started feeding the ball further on their side of the scrum.[22]

Opposing hookers gave up trying to hook for the ball and instead focused on adding power to the pack's drive. This turned the scrum from a hooking and pushing contest into one of power pushing, which led to a rise in collapsed scrums.[22]

In 2007 the scrum law was amended to a four-step, "crouch", "touch", "pause", "engage" process in an attempt to control scrum engagement. Prior to this, there was no obligation for each prop to touch the opposing props shoulder, and the distance between the two front rows was often larger. The new rule fixed the distance between the front rows and as a result cut the force of impact from the engagement.[23] The reason for the rule change was to reduce the number of serious neck injuries to front rowers.[24]

The IRB made further changes to the laws in 2009 in an attempt to eliminate uncontested scrums. Beginning in the 2007–08 season, the IRB sanctioned a trial of the following new rules in France:[25]

Matchday squads would be increased from the standard 22 to 23, with two props and one hooker required among the substitutes. If a team lost all of their available front-row substitutes, uncontested scrums would take place, but with that team having to play with 14 players instead of 15, eliminating any possible advantage of uncontested scrums. In the last season before the trial (2006–07), 145 matches in the country's three top divisions (Top 14, Pro D2, and Fédérale 1) ended with uncontested scrums. In 2007–08, only two matches out of 994 in the same divisions finished with uncontested scrums.[25]

In July 2009, the IRB authorised all of its member unions to adopt these changes in competitions that they completely control, and also allowed these changes in transnational club and provincial competitions (such as the Magners League and Super 14) with the permission of all unions involved.[25]