Unfortunately, WEP is still present in the world. There are legacy systems and devices in certain environments that can only do WEP, plus a number of networks that have no one interested and/or knowledgeable enough to update. Like many advances in technology, phasing out the older technology takes time. Look at IPv4 vs. IPv6 after 20ish years and tell me which is still predominant.
That being said, WEP is no longer viable in modern 802.11 networking. Not only is WEP not viable in modern 802.11 networking, neither is TKIP (was initially used as part of WPA certification). Since the release of the 802.11n amendment to the standard, the use of either requires that devices disable the use of HT or VHT data rates.
In other words, the use of WEP or TKIP causes a modern 802.11 network (i.e. 802.11n or newer) to function little better than an 802.11a/g network. While you do pick up some of the advantages of newer standards, the performance (which is the typical driving force for people to upgrade) is negated.
But all that aside, I have to point out that Wigle's stats are a bit "flawed" unless you actually understand what it is you are really viewing. Wigle is a large, user collected database of information. However, as far as I know, they do not age out old data for a number of reasons (for instance, just because someone hasn't recorded updated information on a network doesn't mean it isn't still present).
So what you have is a large number of networks present in their data that are not present in the real world. If you check many of the WEP entries, they will not have been updated in 5 or more years. Many of these are likely gone or replaced.
In the graph on the Wigle statistics page, they are simply showing the percentage of their database entries that are using the respective technologies. They are not showing the actual technologies deployed in the real world at present. The shown decline of WEP is largely due to new networks being added to the database that are not using WEP, rather than WEP networks being removed from the database. Pulling from the Wigle.net API, these stats may present a more accurate picture of the decline of WEP:
Updated since 2014
Updated since 2016
Updated since 2017
As you can see, while WEP is still certainly present, the real world statistics of WEP being in the wild is much lower than the 6-7% number to which you were referring.