It's not that sequential access is difficult; it's that random access is much more efficient. But obviously IKEA doesn't want you to be efficient in finding what you're looking for.
The catalogue shows you the goods but then when you go to pick them up you are pushed around the entire showroom (with the hopes that you'll see things that will go well with what you've picked out already).
The showroom's purpose for a customer is to show them things before they buy (or alternatively to give them ideas and fashion cues). If I want to buy a bed though I don't really want to look around cupboards, kitchens, children's toys, etc., etc., I want to look at beds.
From the companies POV the showroom is to show you things. For IKEA it is to show you as many things as possible with the hope that some of those things will look attractive to you. If you go in for one thing they are doing their best to ensure that you leave with other things too.
There's also a consideration of sunk costs. The showroom takes about 45 minutes to navigate, if you go around and don't buy anything it feels like you've failed somehow. The idea is that if you buy something then you haven't wasted the time. Indeed there are no exits except through till lanes. You are a customer, you will buy, leaving without buying is made as hard as possible.
They have alarms on inactive checkouts! Sneak out without buying and you set off an alarm; they're good at what they do.
IKEA is one of my favourite places actually. We go there for coffee if we're in the area - I really enjoy looking for the clever little adaptations in their designs that make all the difference.
However, I have found in the last 5 years or so that they appear to have slipped a bit from good value to cheapness.
It is possible to circumvent some of their tactics too, for example going in backwards up to the checkout area and in to the "warehouse".
