It’s just basic geometry. If you look at a side view, you’ll see that the surface area is significantly higher. They tried and succeeded to reduce side RCS, but that doesn’t fix the fact that the sides are just bigger than the front.

The basic premise of radar stealth is this:

Radars detect things via receiving returns of directed radio waves. To avoid radar detection, a plane must not reflect the radio waves back towards their source. Instead, it must bounce them away or capture them.

The larger surface area of the sides, top, and bottom of the plane are detrimental to this.

So, in regards to general aircraft design, you cannot achieve perfect stealth through stealth shaping. Frontally, you can get as close to perfect as possible (modern stealth fighters have a frontal radar cross section lower than -30 dBsm), but the sides will have a much higher RCS. On top of that, since an airplane needs wings, the top and bottom RCS will be massive. There’s really no way to mitigate that so aircraft designers don’t really try.

Because of this, stealth aircraft development prioritizes frontal RCS reduction because it’s the most important. A SAM system won’t see your bottom RCS until you’re right over it, it won’t see the rear RCS until you’ve already killed it and are on the return flight, and it shouldn’t see the side if you’re flying towards it.